Masked move means you compare a bit mask to a data structure.
Say you have an output in a word for a 16 point input as follows:
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Now, you only want (reading from right to left) the first, third and fifth bits. Rather than copying each bit to where you want them, you can used masked move.
So, your mask would be something like:
0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 1
Then you would move the first, third and fifth bits to the destination word, while leaving the rest of the bits in the destination word untouched, meaning the 0 in the mask says "don't overwrite me in the destination."
Now, you will likely be dealing with DINT since the Logix platforms are a 32 bit processor meaning they store their memory, move it and process it in 32 bit chunks.
Hope this helps, and let us know if you want further explanation. You can also highlight the instruction in your ladder and press F1 to bring up a help file.
EDIT: Also something that confused me when dealing with any data manipulation instructions in 5k is the length. Your length is how may "words" you are using. So if you are moving one address, such as in the above example, your length would be 1.