If this is ticked, WICKS will make the card FAT32 bootable, that is the camera will automatically boot CHDK (or SDM) when switched on.
Note that only cameras introduced since January 2011 can be auto-boot from a FAT32 partition (and thus do not need more than one partition, regardless of size). For a list of such DryOS R47 or later cameras see the bottom of the list on this page. Earlier cameras require a FAT16 partition to be auto-bootable.
Note that only a FAT32 partition can be made bootable with this option. If the current format is not FAT32 and Create Partitions is not checked then WICKS will refuse to make the card FAT32 bootable.