You select your model in editing mode and then and then go to Options, and then click on Info. It will tell you how many polygons, edges and faces your model has. It gives you an idea of how many lines, etc, the computer has to draw to make your model come out as a picture on the screen. If you're playing a game the computer has to constantly redraw your model while the game is being played. The game is basically a loop that plays over and over again, redrawing the scene many times each second, so it looks like objects, cars, characters, etc, are actually moving around. That is a lot of computations per second, and the more defined the models are, the more computations must be performed. If the program can't keep up with the numbers the game, or whatever program, will start to run slowly, and if it gets too out of whack it just "crashes" altogether.]
So, you want to try and make models that are low in polygons, etc, if you're going to be using it for games.