This bind is actually made up of three different console commands. Each command is separated by a semi-colon ( ; ), this tells the game to treat them as such.

The first command, 'bind mwheelup +jump;' binds the +jump function (makes your character jump) to the 'mwheelup' key (the key that is fired when you scroll up with your mouse.

The second command, 'bind mwheeldown +jump;' is similar to the first, it tells your character to jump (with the +jump function) when the mwheeldown key is pressed (when you scroll down with your mouse).

The third and final command, 'bind space +jump' re-binds the space bar ('space') to the jump function ('+jump'). This is so you can still use the spacebar to jump.