You can use pro tem as an adjective or an adverb — it basically means "for the time being" or "for now." If you're using your friend's laptop pro tem, you're just borrowing it temporarily.
If you're elected secretary pro tem of your school German club, you're just the acting secretary, filling in for a while until a permanent one is elected. Similarly, if the US president is seriously ill or incapacitated, the vice president becomes the pro tem president. Pro tem is short for the Latin phrase pro tempore, "for the time being."