When you are removing a key if one or both of these white plastic pieces come off the key board, don't worry. They are called key carriers and are very easy to put back on.

If one part of the carrier is still in the keyboard, remove it so you have both in you hands.

Look at the second picture and align the two piece then push them together. There should be a soft clicking sound.

In the same alignment as the second picture, place the carrier in the spot on the keyboard where it came off and press firmly into place.