orphan files

what is an orphan file? how do you fix this

An “orphan block file” is a data fragment that has been found in either Audacity’s temp folder or the project _data folder that Audacity does not recognise as part of the project. The Audacity warning message about orphan block files occurs when orphan block files are found in the project _data folder - I presume that this is what you are asking about.
See here for a full description of Audacity Projects: http://manual.audacityteam.org/o/man/audacity_projects.html

Orphan block files may occur if Audacity does not shut down cleanly (if Audacity closes or crashes before it has had time to clean up its temporary files).

The safest thing to do if you see a warning about orphan block files is to use the default option (“Continue without deleting”) and check that the project plays correctly in its entirety. Then save the project using “Save As” and save the project with a new name. The new project will be saved without the orphan block files.

If the project does not play correctly but has gaps in the playback, then it “may” be possible to recover the missing data by importing the orphan block files. This tends to be tricky, and unless it is a really important recording, not worth the effort.