What a rating is
A single number for how you are playing at the moment, worked out from who you beat and who beat you - not from how many matches you won.
Why it moves the way it does
Beating somebody rated well above you moves it a lot. Beating somebody well below moves it barely at all. Losing to somebody below you costs more than losing to somebody above. That is the whole idea: it measures the standard of your results rather than the quantity.
Provisional ratings
A new player's rating is uncertain, and it is marked provisional until there are enough results to be confident about it. A provisional rating moves in larger steps, and settles down.
When they are worked out
Once a week, on a Monday morning, for the week that has just finished. A match played on Sunday night is in that week; one played on Monday afternoon waits.
What never affects it
A walkover never moves anybody's rating - nobody played. A retirement counts for the result and not for the rating.