Just like Sigma Notation, except that you multiply each term times the rest instead of adding.
To be more technical, it is typically shown as something like:

  n
-----
 | |  k
 | |
k = 1

...which would resolve to something like n!. This gets used just as much in number theory as Sigma Notation, and for simplicity's sake, I'm going to denote it in all my future write-ups as Π(condition; term). The above example would be Π(1<=k<=n; k). You also might see something more exciting, like Π(m is a natural number and p_m is prime; (p_m)^a_m). This is the prime factorization of a natural number (besides 1).