Contradictions are two things that depend on one another to exist but they also exist in opposition to one another. A lot of people seem to lump any logical contradiction of capitalism, but that's not quite right. Logical contradictions are easier to grasp and more intuitive.

Some contradictions are buyer/seller, production/consumption, worker/boss, use value/market value. But it doesn't have to be strictly economic. USA/China is a contradiction. That's why it's important to try to understand the concept of a contradiction more so than memorize a list of them from Kapital. There are contradictions all around us, some haven't been found or analyzed much. They're present in all things, even yourself. If we achieve communism it will have its own contradictions that must be addressed. At any given time there is a singular contradiction above all others. The dominant contradiction changes over time. This is why we must analyze our present world and try to understand its contradictions. Some people get too caught up in history and try to analogize past contradictions to now or assume a past dominant contradiction must be the present dominant one. USA/USSR was a contradiction, but it no longer exists. It is different from USA/China despite similar superstructural rhymes ie cultural cold war and similar foreign policy towards them.