Pop one value from the stack, push it back on the stack and also store it in one of 4 or 256 registers which number is specified in the tag (0, 1, 2 or 3 only if not in a Declare Function (V7). I tried other numbers and they don't work in SWF version 6 or older.) Until set a register has the value undefined. The value of a register can be retrieved with a Push Data action and the register type with the matching register number.
(To be tested) It is likely that trying to read a register which is not legal in a Declare Function (V7) will generate an exception (Yes! A Throw!) but I wouldn't be surprised if you just get undefined.