QUOTE
I'm on battlenet, playing Helms Deep with some other people, I'm the Uruks and The other Uruk tells me to mass. I don't even know what massing is or how you do it.
In many StarCraft games, there is something called 'mass'. The mass command usually takes the form of a unit (usually a civilian, SCV or observer, occasionally a probe or dark templar) and a set of beacons. Each beacon corresponds to a certain place on the map, and beacons that correspond to players' bases often appear in that player's color. The map also contains a set of triggers which look like this:
Affects:Force 1
Conditions:Current player brings at least 1 MASS ATTACK to [mass trigger location]
Actions:Move all MASS ATTACK belonging to current player at [mass trigger location] to masser spawn
Issue order to all any unit belonging to current player at battlefield: Attack to [mass destination]
Preserve trigger
It has one trigger like this for all [mass trigger location]/[mass destination] location pairs. The result is that when you put the unit on the beacon, all your guys will start running towards a particular place. Mass was invented as a means of issuing orders to many units at once, because UMS maps often have way more units than melee maps do and telling them to attack move 12 at a time just isn't fast enough.
Whether that version of Helm's Deep has true mass or not I don't know, although I would assume it does. Look around for any sort of control panel with units and beacons, there should be one there.