Like most words, “racism” has multiple definitions. If you only know one usage, then the concept of “reverse racism” doesn’t make sense. Let’s look at dictionary.com:

racism

noun

1. a belief or doctrine that inherent differences among the various human racial groups determine cultural or individual achievement, usually involving the idea that one’s own race is superior and has the right to dominate others or that a particular racial group is inferior to the others.
2. Also called in·sti·tu·tion·al rac·ism [in-sti-, too, -sh, uh, -nl , rey, -siz-, uh, m, -, tyoo, -],. a policy, system of government, etc., that is associated with or originated in such a doctrine, and that favors members of the dominant racial or ethnic group, or has a neutral effect on their life experiences, while discriminating against or harming members of other groups, ultimately serving to preserve the social status, economic advantage, or political power of the dominant group.
3. an individual action or behavior based upon or fostering such a doctrine; racial discrimination.
4. racial or ethnic prejudice or intolerance.

These are all clearly related, but they are not the same. “Reverse racism” does indeed fall under #4 (“racial or ethnic prejudice”). #4 is probably the most common definition when used colloquially to refer to an individual’s actions or statements.

Definition #2 is more broadly used when discussing matters of public policy and legal issues, which is where you are likely to hear “reverse racism”. The key point of institutional racism (#2) is that it is part of a power structure — there is a group in power that the policies serve to support and strengthen, and there is a group lacking power that the policies serve to oppress and weaken. “Reverse racism” in this context makes perfect sense: it’s reversed to support the oppressed group instead of the powerful group.