its an archaic tradition that involves "cleaning" the wound by orally draining the blood.
Its not so popular today:
>The ancient method of performing metzitzah—metzitzah b'peh, or oral suction[37][38]—has become controversial. The process has the mohel place his mouth directly on the circumcision wound to draw blood away from the cut. The majority of Jewish circumcision ceremonies do not use metzitzah b'peh,[39] but some Haredi Jews use it.[
https://en.wikipedia.org/wiki/Brit_milah#Metzitzah