On the extreme end, a video can be totally fake. Slightly less extreme, a video might be "fake" in that it's an actor playing a role, and the words they're saying belong to the character and not the actor. More commonly, a video might be taken out of context, so that the apparent meaning of a statement in isolation is the opposite of what it originally meant. (Compare "I hate group X" to "I think people who say 'I hate group X' are jerks.") Modern politics is effectively a distributed indexing system for exactly these most ambiguous cases, coupled with large media apparatuses for broadcasting them.

And these are the problems with the simplest sort of fact, "A said B." That's not even the majority of misinformation out there. We usually have to contend with even worse ambiguities, like whether A "caused" B, or whether A "supported" B, or whether A "criticized" B.

In everyday life of course, these problems are solvable. We find ways to punish people for acting in bad faith, and the incentives mostly line up. But that is very much not how things work in politics or in law. People have large incentives to seek out ambiguous cases. These edge cases become the typical case.