Uh, matrix cube? What's that?
You may have heard about datacrons: cubes marked on the game world by a colored light shaft, usually in out of the way or hard to reach places, that you can collect. Most of them give you a small, permanent stat boost, like +2 Strength or +4 Endurance). But a few of them are matrix shards. These sit in your mission items inventory until you go to the matrix assembler (there are one in the Jedi Temple Ruins on Coruscant and another one in the Dark Temple on Dromund Kaas) and make a matrix cube - which is an equippable Relic.
Depending on your level and which matrix shards you use, you can get different cubes. The one you can get at level 50, with the stats appropriate to your class, is actually a very good PvE piece of gear.
You can find how to make the matrix cube relevant to your class here:
You'll probably need a guide to find the datacrons. For instance: