First, this is base 2 arithmetic, also called binary arithmetic. The columns go (right to left) units,2s,4s,8s,16s... just like base ten, which is units,10s,100s,1000s - we multiply by the base to move left a column. When we subtract a 1 from a zero it can't be done (same in base 10) and so we have to borrow 1 from the 2s column. So, 2-1=1 or in binary 10-01=01. We carry on subtracting each column until we have an answer.
For more information google binary arithmetic.
Mike.