Binary number system is has 2 as its base. The way we have digits 0-9 in a decimal system and 10 is the base we have digits 0 and 1 in a binary system whose base is 2.
In decimal a number is represented as follows using the positional values which are powers of 10.
145 1*10^2 + 4*10^1 + 5*10^0
Similarly a binary number is calculated as follows using the positional values which are powers of 2.
1011 1*2^3 + 0 + 1*2^1 + 1*2^0 8 + 0 + 2 + 1 11 in decimal.
0 
1