The inputs (A3, A2, A1, A0) represent inputs that can be high or low (0 or 1). A3 corresponds to 4th bit in binary, A2 corresponds to the 3rd bit, and etc. For example, going bottom to up, 0000, displays the "0" output on the 7-segment display, 0001 displays a "1", 0010 displays a "2", and 0011 displays a "3". This continues using binary, and goes up to "9".