“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.