This is a mux realized from 3-state buffers and AND gates. "Sel1" and "Sel2" are the control inputs (selection inputs). Note that Sel1 is sometimes called "more significant" than Sel2 is "less significant". If there were more control inputs, then Sel3 would be written to the right of "Sel2" and be even less "significant", etc.
Note that the subscripts on the In inputs indicate the decimal value of the binary control inputs at which that input is let through.
The labels 00, 01, 10, and 11 show what the turn on code is for the and gate and the 3-state buffer it connects to.
== Summary == This is a mux realized from 3-state buffers and AND gates. "Sel1" and "Sel2" are the control inputs (selection inputs). Note that Sel1 is sometimes called "more significant" than Sel2 is "less significant". If there were more control inputs,