What do you mean by an emulated game? or What is emulating a game?

Ok so i have heard that an emulation program lets you play a game from anothers computer .is it true?But what program should i use for it? but if you play an emulated game from anothers computer dont you have to pay or what will that person do when you play the game(emulated)? And do i need to have a performant computer or a good monitor and a good internet?