"region UNPLACED_SECTIONS is full" error message

I'm getting a "region UNPLACED_SECTIONS is full (ARM Flash Debug/led.elf) .foo" error message when linking, what does this mean?

This message indicates that you have a .foo section in your program, but it has not been placed by the linker.

Make sure that you specified where you want to the .foo section to be placed in the section placement/memory map file.