what do I see under the oil cup?

It is something new to me since this is the first time I have a car like that.
I am looking under the oil refill cup and I see something turning while engine is running instead of valves....what is it?

Overhead cam drive.

Overhead cam drive.
thanks

should i check something there before i buy the truck?

Yes check over the entire truck, ask for service records or get a pro to check it out.