Wikipedia
M-Lok is a free licensed accessory mounting system developed by Magpul Industries that allows for direct attachment to negative space mounting points. It is a competing standard to Keymod in regards of replacing the MIL-STD-1913 picatinny rail in some applications. Compared to the picatinny rail system, M-LOK and KeyMod enables the user to have a slimmer and smoother handguard with accessories mounted only where needed, whereas a picatinny handguards typically will have picatinny rail slots for its whole length resulting in a larger handguard diameter with sharp corners.
The M-LOK system can be seen as a further development of the Magpul MOE slot, but the two are not compatible. Newer M-LOK accessories can be used on older MOE slotted handguards if an adapter plate is used, however there is no adapter available for using older MOE slotted accessories on newer M-LOK handguards.