Yes and no. Yes you can work on files while a backup is running, but no files your working on will not be updated. We'll gloss over most of the OS design theory and just say that if you open a file, the OS locks it so only that one program can manipulate it. This is necessary because otherwise you could be editing some file, I come along and make some changes while you're working. Maybe you save, then continue working, then I save, overwriting your changes, and keep working, then you save again overwriting MY changes, etc. There has to be some way to control the chaos. That's where the file lock comes in.
So those files will be skipped by the backup program until the next go around. At best, they can get read-only access and then copy whatever the file looked like after the last save, but most likely if a backup program encounters a locked file, it's just going to skip it and move on.