“Who” is a subject. “Whom” is an object. But in our fast-paced world, there often is not enough time to figure out whether something is a subject or an object, especially if you forgot how to do that.
We often advocate a sort of test, where you flip the sentence around to determine whether you would use “he” or “him” in its place. If you would use “he,” it’s “who”; if “him,” then “whom.” Sexist though it is, it often works.
Take, for example, the common police story. “Police described the suspect as armed and dangerous” is pretty straightforward. But take the “armed and dangerous” phrase and put it elsewhere in the sentence, and confusion can arise. “The suspect, who/whom the police described as armed and dangerous …” If you make that phrase more like the original sentence, it becomes clear: The police described him as armed and dangerous. So the grammatically correct phrase should be “whom the police described.”
Read more about the conundrum here.