What does the $\prod$ symbol mean?

The symbol $\Pi$ is the pi-product. It is like the summation symbol $\sum$ but rather than addition its operation is multiplication. For example, $$ \prod_{i=1}^5i=1\cdot2\cdot3\cdot4\cdot5=120 $$ The other symbol is the coproduct.

The uppercase Pi $\prod$ symbol stands for the $product$ operator throughout mathematics, just as the uppercase Sigma $\sum$ symbol would describe the sum operator. Think of the following analogy alliteration:

Pi is to a Product ... as Sigma is to a Sum.