to signal the coming of something
It means that you should draw a circle around the individual words that are signal words... that signal the reader about something. Depending on what type of signal words you are working on, these could be illustrations to the reader that there is more coming, that something important is coming, or signals that tell you what the author thinks... like perhaps they aren't sure of what they are saying. Please see the related link for examples of different types of signal words.
A portent, an omen, or premonition.
If someone says something is coming down the pike they mean its coming up soon
in radio language, it mean that everything's is clear(the signal is coming through strong and clearly)
There is probably something not bolted correctly and is coming loose
you are ready to do something thats coming for you
The AT check signal light means that there is something is wrong with the transmission. A slipping transmission can be the cause of this.
It mean coming up with a plan to go searching for something.
you are frightened something awfull is about to happen
"In the wind" in that context would mean something that was coming in the future
There is something coming your way, normally a ball.
coming directly from actually experiencing or seeing something.