Site swap is a numerical notation that describes juggling patterns, in particular the heights of each throw.
For example, the pattern "51414" means that
the first throw is a "5" - it goes the same height as when you juggle 5 balls in the standard "cascade" pattern. If that throw was made with the right hand, the next throw, the "1", will be made with the left hand. Next comes the "4" , thrown from the right hand, then the "1" thrown from the left hand, and finally the "4" thrown from the right hand. The next throw that happens from the left hand is another 5 (we repeat the number sequence over and over continuing to alternate hands).
Helpful things to know.
A "2" throw is really not a throw at all; it's just a hold for a beat.
A "1" throw isn't really a throw either; it's merely a handing of the ball to the other hand.
A "0" is a nothing - no throw, no pass, no ball - for a beat.
A regular seven-ball cascade is called simply, "7", for it just repeats over and over the same kind of throw.
Any even numbered throw ends up in the same hand whereas any odd numbered throw ends up in the opposite hand.
Interesting mathematical property of site-swaps:
Taking the average of the numbers of any siteswap pattern will always give you the number of balls juggled in the pattern. For example, take "51414": the sum of the numbers 5+1+4+1+4 equals 15. Divide that by 5 (the number of digits) and we get 3. So we know that it is a three-ball pattern
My favorite site swap patterns are:
441 (try with Mills mess in two or with Mills Columns)
531 (throw the 3 five high and claw it)
For more info on sideswaps check out: http://www.juggling.org/help/siteswap/