The common arithmetic operation found in any programming language, OP_ADD works exactly as you'd expect. It requires at least two inputs, without which it'll faill out. While handy, none of the common scripts use op_add for transactions.
OP_Code Walkthrough
Adds the top two items on the stack and pushes the result onto the stack.
Pop top item
Pop top item
Add both items to create a new item
Push new item