Critical Race theory is the belief that, basically, all of history is, more or less, a history in which white people did their best to oppress minorities, enslave them, and everything else. That they're basically guilty of all crimes and are nothing more than 'white devils'.
Not only is this not true but it does it's best to change the motivations of things that are true to to be entirely racist and, in it's most extreme form, will even claim that everything done by white people is either stolen from other people groups, made purely out of greed/hatred/something else negative, and that white people are basically uncivilized barbarians who only seem good because they managed to re-write the history books.
Basically, imagine a marxist world view in which the wealthy do nothing but oppress the minorities and everything done ever is done on the backs of oppressed people except swap out 'rich' for 'white' (though they're often the same) and swap out 'the working man' for 'minority' and you'd have more or less the same outcome.
The problem is that it not only teaches untrue facts about history (such as re-framing westward expansion/divine mandate to be basically an attempt to genocide native americans) even in 'true' facts it assumes the worst about white man and does it's best to ruin any positivity. It also usually either assumes the best or flat-out ignores any negative things done by other ethnic groups. This tends to be followed up with additional things to reinforce the thing like making a dorm exclusively for minority groups and other things.