By default, this button locks the autofocus and exposure. It can be reconfigured to lock only one or the other, or other things. See the "f4: Assign AE-L/AF-L Button" section of the manual (page 200 in my copy). A fuller description of the focus lock is given in the "Focus Lock" section of the manual (page 57 in my copy).
If you need a manual you may download a copy from here.
Answers & Comments
By default, this button locks the autofocus and exposure. It can be reconfigured to lock only one or the other, or other things. See the "f4: Assign AE-L/AF-L Button" section of the manual (page 200 in my copy). A fuller description of the focus lock is given in the "Focus Lock" section of the manual (page 57 in my copy).
If you need a manual you may download a copy from here.