Greyed out squares in layers under the eye

[caro J]

hi,
When you are in layers you see an eye, a padlock and below is the mystery tool....the greyed out square of tiny squares.
if you touch it it goes black, which presumably means something is turned on...but what is it and what does it do?
Thanks so much....this is a long held mystery for me. is it something to do with opacity which can be set in the palette?
Thanks caro J

[Mr. Kibernetik]

This is Alpha Lock.
When on, alpha channel of the layer is not changed.