Can I use a AT&T iPhone on the go phone at&t without unlocking it?

I'm going to buy a AT&T iPhone on Craigslist and I want to be able to use it on the AT&T go
phone. I don't really know what "unlocking" means so if you could please describe that or tell me I
don't even need it to use it on the go phone that would help a lot! Also if you do know describe how i can put it to a go phone plan!

Unlocking means that you can use a different carriers SIM card in the device and get service. You Can use an iPhone with a GoPhone account for phone and text only. Internet will not work on the network, just on Wi-Fi.

You don't need to unlock it to use as a gophone.
but smartphones can't use data on any gophone plan.
only calls and texts.

Yes you can.