Ping has several meanings in World of Warcraft.
Minimap
Ping usually means to click on the Minimap, which plays a brief sound and highlights the clicked location on party members' minimaps. This can be useful for pointing out flag runners in WSG, pointing out hard-to-see objects like 
[Serpentbloom] in Wailing Caverns, or just plain flipping out and irritating the party.
Latency
Some players of a technical bent might use "ping" to refer to how much lag they have, after the classic networking tool.
Communication
Ping system allows players to quickly communicate with their team without having to use voice or text chat. Holding G + Left Mouse Button opens up a ping wheel to quickly call out commands such as Attack, Assist, Warning, and On My Way. It is also possible to send contextual pings by holding G and clicking an enemy, the ground, or the unit frames.
Patch changes
Patch 10.1.7 (2023-09-05): Ping system added.