Someone dials the number of your pager and the pager then displays their number, which lets you know that they want you to call them. You then make your way to the nearest phone and call them.
They didn't really have cellphones yet, so people carried around pagers, which are pretty much cellphones that only accept the text message "Call me."
That's dumb. What's the point?
My step-dad still has one for work. He's a doctor, so often if he has patients in the hospital, nurses will call or page him to ask him questions, or sometimes he'll get called in to assist with surgery. The pager is the super simple version of a text, as someone else has said!
And the cellphones that they did have were gigantic to the point that you couldn't really carry them. A pager could fit in your pocket.