.tmp-files are temporarly files created by incomplete WordPress- or plugin actions.
These files appear also in general computing.
In this case, they are often created during plugin updates that are performed from the WordPress backend (wp-admin). If the plugin upgrade fails for some reason, or termitated by the user, the .tmp files may not be deleted by the system.
The .tmp files can be deleted safely under normal circumstances – since they are usually the forgotten residue of some incomplete WordPress actions.
However, it might be a good idea to backup the files to a local folder before you delete them, just in case..