Canon is basically a condition that applies to anyone/anything in the primary timeline and allows them to move the plot/Sburb's progress along. Due to Homestuck following multiverse timeline rules there are nigh-infinite timelines where something was done slightly differently than the primary and these timelines are considered non-canonical as they will not be relevant to the main plot progression. This is all made even more muddled by the fact that canonical characters can freely enter non-canon zones and interact with characters that have lost or never had canonical significance. The biggest example of this is the dream bubble segments. One of the quickest ways to become non-canonical is to die as death and time in the dream bubbles tend to freeze characters in their current stage of development, and ongoing character development is extremely important to remaining canonical. This stuff is explored more explicitly in the meat/candy epilogues. Additionally non-canon characters and things can be pulled into the main timeline but as a rule they won't actually be useful in accomplishing anything that would move the plot along.

Lord English is the end of the current universe, the ticking clock that forces the players to actually complete Sburb or be eliminated. He is an inevitability created upon the start of Sburb along the same lines as the destruction of the earth at the beginning of the game. As he is intended to be the end of all things he has basically complete control of time and also is a big green dude that can do DBZ style beams. Really a proper breakdown of his abilities would be a bit pointless cause he's basically that boss with infinite power you're not meant to actually fight in a game.

Paradox space is the name of the somewhat metaphorical plane of existence that allows characters to travel between different timelines and realities such as the dream bubbles. Imagine space, but instead of planets you've got timelines. In that metaphor dream bubbles are kinda like asteroids created when a planet explodes, small fragments of a non-canon(dead) timeline just floating around unmoored from context.

Juju's way more nebulous than most of this from what I remember. It's sort of an elemental evil that's part of Sburb and can possess things to further its own agenda. It's a lot like Bob from Twin Peaks, but with an end goal.

The Green Sun is the powersource for a lot of the more out there elements of Sburb. It's a generator that various Sburb parts use for fuel and will eventually be fully devoured by Lord English as part of his final power up/full universal armageddon move.

The Horrorterrors are extradimentional entities a la Lovecraft that exist in the far bounds of Paradox space. They're not great dudes but are opposed to Lord English and aware that his end goal will wipe out them along with everything else. As such they groom Rose and empower her to ensure Sburb is completed regardless of what rules need to be broken.

The Incipisphere is a dimension deep in paradox space that's basically the game board for Sburb. You're transported there as part of the start of the game and expected to fulfill Sburb's objectives within its space.

The Scratch is basically a "reload saved state" button for when Sburb players know they're fucked. Activating the scratch is what you do to roll back the clock once you've determined there's no way to complete the game in its current state.

Death removes players from canon assuming the player in question isn't god tier. Dead players can still be interacted with in the dream bubbles but once there won't be able to effect the main plot and generally can't develop as people any further than at the point of their death.

The time system is basically the most fiddly version of multiverse time theory you can imagine. Every choice splits off timelines but only one timeline is considered canon. Every moment of Homestuck splits off hundreds of doomed timelines that will be destroyed.

The power system is overcomplicated and something with a ton of fanwank around it to the point I have no clue what in my head is canon and what is pure theory. The quick summary I remember is that characters get titles that determine their powers in the format [Title] of [Aspect]. The aspect determines what they have power over and the title determines how they're able to wield that power. To give the best example I can think of in the text is the juxtaposition of the Heir of Void and the Thief of Void, the Heir basically grows into their aspect and eventually is noted as being "less there" inherently than other people due to their relation to nothingness, whereas the Thief is able to steal from void, retaining their full presence while also making use of the aspect in a more action oriented manner. Aspects tend to be either passive or active in this way.

Sburb is reality's method of reproduction. Each reality, which includes all timelines/paradox space constructs in its remit, eventually must be wiped away for a new reality to be created. Sburb is created in a reality once its time is up and once activated will directly start the dominos that end all things. The players must finish the game in order to create a new reality which will eventually in turn be wiped out as part of the cycle.