It's a race relations viewpoint that is supposedly being taught in public schools and diversity training. Or the right wingers are trying to preempt it from being taught in public schools. I'm not sure which it is, probably both. I know UT and other universities have courses based on it.
From my limited understanding of CRT, it argues that policy and actions that don't have racist intentions but produce disproportionately negative effects for POC. An example would be single family zoning; it's a racist policy because it tends to limit the number of minorities. So it basically expands the definition of "racism" beyond what some (including me) conceive it to mean.
But that concern is really just a semantic one. Another common objection is that CRT looking at almost everything through a racial lens is counterproductive in various ways, including nurturing resentment and guilt of whites, and all for things they didn't even intentionally cause.
Note that I haven't studied CRT and the above is based on my limited reading of commentary re: CRT, so anyone interested should do their own research. Hopefully this post is at least a little accurate and useful to those who aren't or weren't sufficiently interested.