Siteswap is the expression of juggling pattern by numbers.

0 means that Your hand is empty (for one beat).
1 means that You hand a ball to the other hand.
2 means that you hold a ball in Your hand (for one beat).
3(5,7...) means that you toss the ball to the other hand at the same height as 3(5,7...) Cascade.
4(6,8...) means that you toss the ball to the same hand at the same height as 4(6,8...) fountain.

If you average the numbers, You will find how many objects You will juggle.
(For examle: 5524 - 5+5+2+4 => 16/4=4 balls.)

The first number is for the first hand, second number for the other hand, third is for the first hand again and so on.

Vanilla Siteswap doesn`t make a difference between Mills Mess or Cascade and you cannot write down Multiplexes or synchronic siteswap.

A few examples:
3 or 333 - 3 balls cascade.
5524 - pattern "three arround one".
24 - right hand holds one ball, left hand juggles 2 balls.

With more generous notation we can write down multiplexes and synchronic siteswap.
Multiplex can be 24[45]. Synchronic siteswap can be the box (4,2x)(2x,4).

If You have a siteswap and want to see it, try some simulator program for example "Juggling lab".