Responding to a legitimate criticism by pointing to a different (often unrelated) problem — usually one involving the critic or their allies. The goal is deflection: if everyone has problems, no one can be held accountable.
Classic Cold War tactic: when the Soviet Union was criticized for human rights abuses, state media would respond, "But what about racial violence in America?" Neither claim refutes the other. Both can be simultaneously true. But the rhetorical effect is to make the original criticism seem hypocritical or selective, muddying accountability.
Whataboutism exploits our sense of fairness. If a criticism is being applied selectively, that's a legitimate concern. But the mechanism is abused to suggest that all criticism is just political targeting, making consistent accountability feel impossible. It also triggers a defensive response in anyone who identifies with the group being raised as a counter-example.
"But what about [different group/person/event]?" in response to a direct criticism, especially when the counter-example is not causally related to the original claim.