What is a voting rights violation?

Committing enough of them can result in a state being placed under pre-clearance.

But what exactly are they legally?

I'm not completely sure, but I'd imagine reading through these links could help: 

- 52 U.S. Code § 10101 - Voting rights.

- 52 USC 10301: Denial or abridgement of right to vote on account of race or color through voting qualifications or prerequisites; establishment of violation.

- Justice Department: Voting Section.