"Whataboutism" is an old Soviet tactic used to deflect criticism. If someone accused the USSR of human rights violations, the response would be "what about the violations in China or Africa? Where is your concern for those?"
This has at least 2 ways of putting an accuser on his or her heels. First, there is the implication the accuser is not really concerned about the wrongful act, but merely has a vendetta against the accused. Second, it can force the accuser to try to distinguish between levels of bad acts. (i.e. "I'm not going after China because its atrocities are different from Russia's....somehow.)
"Whataboutism" is a deflection tactic, but the users often don't even realize they are doing it. One person's bad conduct is not excused just because other people got away with the same bad conduct. This, however, begs the question: what if the conduct isn't bad after all, because it is commonplace? (And this is a question that is highly relevant given that we're talking about a bunch of corrupted clowns in Washington D.C.)
If we are discussing Trump's documents, deflecting with "but Hillary's emails" is 100% whataboutism. You are basically accusing the other person of being upset about Trump just because he is Trump and not because of any potential crimes, which does nothing to advance the conversation (no matter how true that actually turns out to be.) However, if you are able to show that Trump's actions are similar to other former presidents, then you are defending Trump rather than attacking the accuser.
(Sorry if this post feels like rambling; it basically is. Just got home from a 4 hours drive in thunderstorms and my brain is barely in neutral.)