"Milkshake" by Kelis
Within the context of Kelis' 2003 song "Milkshake," milkshake is a metaphor for blowjobs. If you replace the word "milkshake" for blowjob or blowjobs it becomes clear.
"Milkshake" by Kelis ~ Verse #1

'My milkshake brings all the boys to the yard,
And their like
It's better than yours,
Damn right it's better than yours,
I can teach you,
But I have to charge'

EDITED:
'My blowjobs brings all the boys to the yard,
And their like
It's better than yours,
Damn right it's better than yours,
I can teach you,
But I have to charge'