If you’re in a meeting and make a comment that is ad rem, what you've said has some relevance to the matter being discussed — as opposed to being completely beside the point or going off topic.
Ad rem is borrowed directly from Latin. It literally means "to the thing," or "to the matter at hand." It's usually used in more formal contexts, such as business meetings, legislative debates, or legal arguments. But you could use it to impress your friends next time you're having a discussion: "Let's keep our comments ad rem, please!" When debating an issue, an ad rem argument is relevant to the facts, as opposed to an ad hominem argument, which is an attack on an opponent's character.