that's a refund for all of your old skill experience and modules.
when you select a mech and hit skills, hit ADD SP at the top right, then select USE AVAILABLE SKILL POINTS and move the GSP slider over until you have 91 points available ( maximum )
then you can use those 91 points to unlock nodes on the various skill trees. for example, stalker you probably want to unlock a bunch of SURVIVAL nodes to buff armor and maybe mobility to increase turn & torso speed, then a bunch in Firepower depending on your build. the "reduced laser duration" nodes are pretty good if you are running standard or ER lasers.
catch me on TS if you want a walk through the first one