What does it mean when the catcher is 'flashing signs' to the pitcher?

What does it mean when the catcher is 'flashing signs' to the pitcher?

Alan Jaeger (Pitching Coach / Founder) gives expert video advice on: What is a 'catcher' in baseball?; What are the defensive responsibilities of a catcher in baseball?; What are some tips for a catcher to achieve the ideal stance in baseball? and more...

The catcher is giving signs, quite simply, by using his fingers. Depending upon what finger he puts down is the kind of pitch that he's relaying to the pitcher to throw.