It's the sound a girl makes when she's giving good head.

It means giving / receiving head

The double Glock Glock combo is a oral sex position usually used by the female to the male

The double Glock Glock combo is a oral sex position usually used by the female to the male

when either female/male is giving that scrumptious head and sticks their hand so far up your ass they can jerk you from the inside.