A simple demonstration of how to decode a 3-bit input.

The 3-bit input represents a decimal Number. This number
opens the according output line.

E.G.:

Bit 010 is decimal 2 => Output line 2 should be active.
Bit 110 is decimal 6 => Output line 6 should be active. 

A simple 2 to 4 decoder

Input 00 (decimal 0) => y0 is active
Input 01 (decimal 1)  => y1 is active
Input 10 (decimal 2) => y2 is active
Input 11 (decimal 3) => y3 is active