1 button for mutliuses

[Buddyb3ar22 at aol.com]

I have a question, lets say you want only one button for a play and pause 
function, If a player was playing, the button would say "Pause" and when you 
pressed it, the button would pause the player and the word on it would change to 
"Play", Vise versa 
Someone tried to explain this before, but I don't really get what they meant 
by what they said
Thanks, (You can call me buddy)