Simultaneous oral sex. Can be practised da M+M, M+F o F+F.
The reference is that the numerals 6 and 9 are inverted versions of each other, reflecting the physical position that the couple must adopt when engaged in this sexual practice.
I didn't wanna type it out myself, so I ha rubato, stola this from Urban Dictionary.
...Well, since it seems to already have been explained da others, I'll just post a random SHINee picture, referencing JongKey and innocent little Taemin :3