A signal is a piece of data that points in a certain direction. A fight might be a signal that a relationship isn’t going to be productive in the long. A high conversion rate might be a signal that people like your product.
Signals are a useful model because people tend to jump to conclusions. Thinking in signals helps with that. If there are many signals that point in the same direction, you can probably jump to a conclusion. If there’s one or two signals, it’s useful to say: “ok, that’s a signal, interesting”.