Binary has only ones and zeroes. In this video, an extended finger is a one, others are zero. Thumb is one. Index finger is two. Middle finger is four. Ring finger is eight. Pinky is sixteen.
I apologize in advance for four. (Unless you are British, in which case I apologize in advance for six.)
Notice all odd numbers have the thumb out. So when counting, the thumb acts like a metronome.
Zero is a fist. All fingers out is 31, because 16 + 8 + 4 + 2 + 1 = 31.
Have fun! Impress all your friends with this super-nerdy party trick!