It's all about the hard points and tonnage.

The newest entry in the Mechwarrior Online dev blog has landed, and this time they talk MechLabs. 

It's a bit too long to get into all of it here, but I'll recap - they talk about how Hard Points and Tonnage work. Hard Points are places where you can fixate weapons and gear, but it's all limited by how much the mech weighs (if it can't move, it's not useful). Each weight class of mech has its maximum amount of armor it can have, and the different types of armor (Standard, Heavy, etc) tally differently on the mech. Modules and camo are also addressed.

Really, though, if you've been a big enough fan of MechWarrior to recognize a lot of these terms, you're going to fit right in - most of it's identical to the tabletop game.