Can somebody explain (and give examples) of "signals" in quant investing? What are those? What does this word mean?
As people in the comments noted, signal broadly refers to a trigger variable that denotes an investment decision. This is normally a boolean variable (i.e. 0 or 1) but could be continuous (0 to 1) or any other range (e.g. -1/0/1 sell/hold/buy), depending on what your execution algo might dictate.
Just wanted to add that this terminology comes from the general field of signal processing, where, broadly speaking, you take noisy information and translate it into meaningful information. In this case, you are taking market data and transforming it into a buy/sell decision.
More specifically on signal processing though, Digital Signal Processing is a somewhat popular subfield for quantitative finance, and there are some interesting work done on this.