This is the 'volume name' of the first partition on the card. If "Create partitions" is ticked, then whatever you type here will be the new volume name. If only one partition is to be created this will hold DISKBOOT.BIN and the SDM/CHDK folders as well as photos and movies.
If "Create partitions" is not ticked, this field will be greyed out.
If two partitions are to be created, this will be a small 16MB bootable partition used merely to hold DISKBOOT.BIN