In
FreeBSD at least, it means Merge from -STABLE or Memory
File System.
Merges from -STABLE don't occur often, but they do occur. The term follows the lead of MFC. If there are two -STABLE branches (a new one, and a sooned-to-be-phased-out one), then sometimes the term MFS is used, where x is the new one, when comitting to the old one after it's been tested in the new -STABLE branch.
Memory File System is similar in some ways to ram disks, except it can be swap backed, and uses VM. They are created with mount_mfs(8).