"Zeitarbeit" means to work for a company for a specified period of time, i.e. not as a full time employee.

Pons translates "Zeitarbeit" with "temporary work/job". Today though, especially in the IT industry, you would more often find the expression "contract work" or "contracting".

"Zeitarbeitunternehmer" could therefore be a "temporary worker/employee" or simply a "contractor".

BUT there might be chance that "Zeitarbeitunternehmer" may actually mean an "agency" for temporary work. This would very definetely be the case if the source word would be "Zeitarbeitunternehmen" - which is an agency that makes contact between outsourcing companies and contractors.
You might want to check the context of your document in case this might apply.

Hope this helps. Good luck.