Please explain the following Verilog code of a D flip flop?
1'b0 is Verilog syntax for a constant value that is a one bit number expressed in binary format with a value of zero. 

A bit value of one would be expressed as 1'b1.

Similarly a four bit value in binary that is equivalent magnitude to a decimal value of 11 could be written as 4'b1011.

You could also express this same value in a hexadecimal notation as 4'hB.