If you use Mac OSX and use Time Machine to keep your system backed up, eventually you will run out of space on your backup volume. The answer is to go into the backup volume and delete the oldest backups. Why the OS doesn't do this for you automatically, is a mystery to me.
Once you delete the older backups, then you will want to empty your trash. Sometimes this is a problem.
Occasionally the trash will end up with a "Locked" file in it. This prevents the trash from emptying and sometime hangs the trash emptying process. Here's the solution...
1. Restart your Mac. This releases files that are in use.
2. Try to empty the trash again. If it fails, telling you that some items in the trash are locked, it will offer the seemingly helpful option of deleting ALL files or only deleting the LOCKED ones. Neither will work.
3. Right click (or left click and hold) on your trashcan icon and you will get the context menu with "Empty Trash" in it. Let go of the mouse button and hold down the [option] key while clicking "Empty Trash".
That should do it. The trick to emptying the trash can of LOCKED files is to hold down [option] while clicking "Empty Trash". Seems to me that Apple would do that in the background when you select either "Delete ALL Files" or "Delete LOCKED Files"... but they don't. So much for "Just Works" eh?