The scientific answer for this is it is the sac in humans that holds the glands for milk for human young to feed from. All animals even humans male and female have them. If you were looking for a more pervet answer you came to the wrong place
What Is A Boob?
Your category shows dogs, but I doubt that this is what you are referring to, I think that you are looking at the slang word for either a breast or a dimwit, someone who is an oaf, a dud, etc. Deragatory term belittling another. I hope no one called you this as I would be offended
It is also a word like 'racecar', spelled the same way front to back or vice versa...lol.
Things on girls on their chest that provide milk for their babies, and or for men lol
Its female thing to feed you baby with
Can also used in reference to a really stupid person, as in "he's a real boob"