Deleted original post - confused mux and demux.
D represents one of four outputs, d0 through d3. Depending on the value represented by s0 and s1, the signal I is passed on to one of the four outputs. The others are usually low (digital). For example, if s0 =1 and s1=1, then the value of I is sent to d3.
Your column for I is misleading, as whatever the input is, it is sent to the selected output. The selected output is defined in the manner described above.