What are those lumps under armpits?

Ugh, I KNOW that this is disgusting, but I HAVE to know! I get them all of the time! My mom tells me that these lumps are filled with deodorant, and when opened, a horrid odor permeates the room. I don't know WHAT these are, I don't know HOW I get them, and I wanna know how to get rid of them! Please help!