A disk defragment is a process of "tidying up" your hard disk by placing all the files together "neatly" in one place, and moving large files together in the same place when they may be scattered in smaller parts all over the disk.
To that end it is a sort of never ending process which could go on all the time, though after the initial tidying up it will have less value if you keep doing it.
So if you stop it then it will already have done some tidying up, and if you start it again it will continue to do a tidying up.
It wont actually start from where it stopped as it will probably re-analyze the disk to decide what needs doing, but it will tidy up the parts of the disk it did not do before you stopped it.