Done or said at exactly the right moment. The action itself might be quite ordinary — a phone call, a joke, a question, walking into a room — and what makes it 'well-timed' is only WHEN it happened. A well-timed joke lands because the room was ready for it; the same joke ten seconds earlier is just noise. So this is praise for a person's sense of timing, not for the thing they did. It sits before a noun most of the time ('a well-timed break', 'a well-timed goal') but you can also say 'that was well timed' after something happens, and many people write it without the hyphen there. In sport and in business news you meet it a lot: a well-timed pass, a well-timed investment. Note it is usually about a small window — the moment would have passed. If something is good simply because it happened at all, you would not use this word. There is a dry, joking use too: someone arrives just as the work is finished, and a friend says 'well timed' meaning the opposite. Tone of voice tells you which one it is.