The number eleven in binary is 1011. When written in the commonly used base ten system, eleven is composed of one ten and one one. Binary is a base two system, and in this system, eleven consists of one eight, zero fours, one two and one one.
Each column of a binary number, when read from right to left, has twice the value of the last column, so instead of ones, tens, hundreds and thousands, binary has ones, twos, fours, eights and so on. To convert a number into binary, start with largest binary value and work down. In this example, eight is the largest binary value that fits into eleven, so there's a one in the eights place. Eleven minus eight is three, which four does not fit into, so there is a zero in the fours place. Two does fit into three, so a one goes in the twos place, and one fits into one, so a final one goes in the ones place.