That's a burr.
Effectively, a tree scab.
Some people cut them off and use them to make tool handles or bowls and cutlery type things. But you shouldn't go cutting them off live and healthy trees.
Not sure what kind of tree that is. The lenticels (the slash like marks on the bark) make me think it's some kind of Poplar (an aspen, perhaps?), but I'm far from certain. Lenticels are also common on fruit trees, but it just doesn't say fruit tree to me. A picture of the leaves would help.