"ad hominem" is a Latin phrase used very commonly in English (just like "ad nauseum", and also "i.e." and "e.g." are abbreviations of Latin phrases). It is, effectively, part of English.
What it means is, in the context of an argument, referring to who made the argument, as opposed to the argument's own referents, in deciding whether to accept or reject it. It is normally considered proof that one has no real argument to offer against or for what is said, if one must invoke "who wrote it" as an refutation or as a reason of non-acting upon it thus in silent approval of it.
Ad hominem is the weakest form of argument. It is an attack or defense on an argument based on who made the argument, rather than based on its merits.
Speculative attributions of motive are ad hominem (an observation of English Wikipedia User Bird) not to mention amateur psychiatry based usually on pop psychology. More seriously, and much more damaging:
Tolerating any form of ad hominem attack (which insiders can get away with but outsiders can't) tends to create a hierarchy of insiders and thus an insider culture similar to that of bureaucracy or academia, where a trusted name can spout nonsense for years without being detected or kicked out, and where untrusted names with proof of their claims get ignored.