First number before the dot is the number of speakers.
Number after the dot is the sub-woofer.
Usually, you have two speakers, left and right, for stereo sound. You could get 4, to have two front and two back for surround sound.
Then you'd often get a sub-woofer, which handles all the deep bass (since your brain can't detect where these deep sounds come from, so you make it separate and put it anywhere).
To sum up:
2.0 = regular stereo speakers
4.0 = surround sound
2.1 = stereo with sub-woofer for bass
4.1 = surround sound with sub-woofer
And you also mention a 5.1. You've guessed by now that this includes a sub-woofer. But it also has an extra speaker in addition to the normal 4.1 setup, which sits right above or below the screen, and outputs dialogue. You don't really want people's voices in a film coming from the left or right of you, when they're on screen in front of you. So you put another one just above or below the screen, making 5.1.