Flashing lights on a parked car usually signal a warning (to other drivers).
He signalled left, and turned the lorry slowly.
He was signalling (= giving a signal) with a red flag.
She signalled for help.
[ + that ] She signalled to the cars behind her that they were going the wrong way.
[ + obj + to infinitive ] The children's mother signalled them to be quiet.
[ + to infinitive ] The children's mother signalled to/for them to be quiet.