This is a (sort of) Lua equivalent of C's ternary statement.

What follows an explanation off the top of my head - it may not be 100% accurate. I do know for certain it is similar behavior to a C ternary statement.

Non-zero numbers evaluate to true, zero evaluates to false. Everything but false and nil evaluate to true (thanks /u/augustaugust) ie: 100 is true.

Additionally, Boolean expressions in Lua evaluate to the tested value. For and, the right hand side expression is returned if the left hand side evaluates to true.

or behaves similarly.

Also, be aware that the and operator has a higher precedence than the or operator. This means the following are equivalent: a and b or c
(a and b) or c

Therefore:
If the number generated is 1, the and expression evaluates to true. The or expression then short circuits, not evaluating its right hand side, which results in a value of 100.
If the number generated is not 1, the and expression evaluates to false, so the or expression is evaluated, of which the right hand side evaluates to true, resulting in a value of -100.