Stupid question, isn't it?
/We all know what rhythm is -- at least we know if when we hear it and feel it. But what is it really -- what makes rhythm tick? Wikipedia defines it as "any measured flow or movement, symmetry" - in other words, it is the variation of the length and accentuation of a series of sounds or other events.
The sun, for example, has a rhythm. It comes up every morning and goes down every evening. Steady. The earth has a rhythm too, as it revolves around the sun. We call that rhythm a "year." Your heart has a rhythm too -- if you take your pulse you'll see it is quite steady. All of nature has rhythm, and so it is quite natural that music has rhythm as well.
We speak of that rhythm in terms of "beats", and we measure those beats using tools such as "time signatures", "quarter notes", "eighth notes", "syncopated notes", "steady notes", etc, etc, etc. Most music is in either 4/4 time or 3/4 time, although there are many other varieties that are not used nearly as music.
To master rhythm a person needs to have a "baseline" by which to measure the notes that are occuring in various patterns.
