With openMosix you can start a process on one machine and find out it actually runs on another machine in the cluster. Each process has its own Unique Home Node (UHN) where it gets created.
Migration means that a process is splitted in 2 parts, a user part and a system part. The user part will be moved to a remote node while the system part will stay on the UHN. This system-part is sometimes called the deputy process: this process takes care of resolving most of the system calls.
openMosix takes care of the communication between these 2 processes.
oMFS is a feature of openMosix which allows you to access remote filesystems in a cluster as if they were locally mounted. The filesystems of your other nodes can be mounted on /mfs and you will, for instance, find the files in /home on node 3 on each machine in /mfs/3/home.