What is MGLS? The use of ELS and CLS before PLL for LS+LL

ELS – Say you have the first 2 layers of a cube done, apart from one “slot” (an edge and corner adjacent to each other usually shown in the DFR and FR positions. When one does ELS, then place the FR edge in, while also orienting all last-layer edges.

CLS – Say you have the F2L done, apart from one (usually the DFR) corner, and you also have the last-layer edges oriented. When one does CLS, they will be placing the (DFR) corner into its correct spot, correctly oriented, thus finishing F2L, all-the-while orienting all 4 last-layer corners. This leaves one to only have PLL left.

If one were to use MGLS in full in a CFOP/Fridrich solve, it would consist of:
Cross, 3 F2L slots, CLS, ELS, PLL

If one were to use MGLS in a Petrus or ZZ solve, where the edges are already (inherently by the basic steps of the methods) oriented, they would simply throw the edge with {R,U} moves, then proceed to CLS.

As far as resources and algorithms go, please see the following: