I am reading up on boron in my notes and there is a certain paragraph that I do not understand

The tendency of $\ce{B}$ to form $\ce{B-B}$ bonds is less pronounced than that of carbon, its periodic table neighbor on the right, to form $\ce{C-C}$ bond, but it is much greater than that of the nearest element in the group, $\ce{Al}$, to form $\ce{Al-Al}$ bonds

Anyone can explained to me why is $\ce{B-B}$ bonds can be formed in the first place?