The tri-link suspension moves the rear upper control arm mounts to the center of the axle housing just above the pumpkin, to a bracket bolted to the top of the pumpkin. This allows the rear track bar to be removed while keeping the axle centered. The benefit is the axle has more articulation so it can follow extremely uneven terrain more easily.
If you're not doing truly extreme trails, you don't need to tri-link your suspension. I managed to make it through the trails in Johnson Valley like Sledgehammer and Sunbonnet with the standard non-trilinked RE long arm suspension though the long control arms liked to get hung up on the rocks.