Many things, but often means your pressure tank is out of air, or if a bladder type, is mis adjusted or the bladder ruptured.
A pump that goes on and off a lot means the air cushion is very small. This is hard on the pump, and better to fix sooner than later. Do a google on 'water logged' and you will find more info than you know what to do with.
Sometimes the pressure switch is on a small little 1/8 inch pipe, and that tiny pipe will fill with sediment/ minerals, and so the pressure switch does not get the proper reading to go on or off.
Sometimes the pressure switch goes bad, can buy a little time filing the points but generally just replace.
I'd kinda go in that order, then call the professionals in my house. ;) if you have real pure water not hard, perhaps you should reverse the last 2 in order.