Technically, it is a reaction to racism towards non-Caucasian people. It includes economic and political benefits in effort to compensate for past and ongoing social oppression experienced by non-Caucasians.
Such oppression ranges from marginalization to physical abuse to childhood emotional scarring to the gym teacher shouting "DO YOU SPEAK ENGLISH" to every Chinese kid he sees. But none of these examples are anywhere near as catastrophic when compared to a reactionary anti-white joke told by a black standup comedian. From this reasoning emerges the term "reverse racism".
With respect to the term itself, "reverse racism" implies that there is backwards logic behind racism against Caucasians, and that it is somehow more deserving of disapproval than just regular racism.
Furthermore, "racism" and "reverse racism" can be interpreted as binary oppositions, where "racism" is the (socially conditioned to be) acceptable state, while "reverse racism" is the unacceptable counterpart.
Reverse racism can also be interpreted as a defense mechanism for "white man's guilt".
"Anyone who points out our race problems is a reverse racist." - the GOP