"Leg Meta" refers to the tactic where targeting the legs of an enemy would be the fastest way to eliminate them. In Tarkov, it is possible to deplete an enemy's health by repeatedly shooting them in the limbs, legs being the easiest limb to aim for. There is no leg armor in Tarkov, thus making limbs unprotected and vulnerable to all types of ammunition. Therefore, it is easy and cheap to dispose of someone using that technique.
Because the developers of Escape from Tarkov don't want to acknowledge the efficiency of this technique, the expression became a running joke, endorsed by the COO Nikita himself.
In earlier game versions, when armor was still in an early stage, "leg meta" used to be much more effective, especially against armored targets, but it's still a valid strategy nowadays.