"1 is leftshifted PD3 places and then bitwise anded with PIND. If the result of the operation in the previous sentence is nonzero, then button hasn't been pressed. Else, the button has been pressed."
For instance:
If PD3 = 00000011 and PIND = 10000000, then the following happens:
00000001 becomes leftshifted by 3 places resulting in 00001000. Next, 00001000 is bitwise anded with 10000000, BUT, since the answer to the anding is 00000000, the argument in the if statement is false, so the button has been pressed.
If PD3 = 00000011 and PIND = 00001000, then the following happens:
00000001 becomes leftshifted by 3 places resulting in 00001000. Next, 00001000 is bitwise anded with 00001000, AND, since the answer to the anding is 00001000, the argument in the if statement is true, so the button hasn't been pressed.