Mixup is shaking your opponent’s block around. Well, not always; mixup more literally means to vary strategies in an attempt to get the opponent to guess what you are doing (and guess wrong). The most basic of basic mixups are as follows, however.
Using throws, overhead moves, and low moves, you attempt to mess with your opponent’s blocking.
Unlike in battle with the AI, you will rarely score a lone attack on a human: they will almost certainly block it if they can see it coming. This is why mixup is an essential skill to have. Competency in it makes a big difference in human vs. human fights. Conversely, it isn’t as effective against the computer (since they have no actual reaction time, mixup with them is simulated).