question about "incentive" from Satoshi's paper

I tried posting this in r/bitcoin but it was instantly removed for reasons i do not know

I was reading Satoshi's Bitcoin: A Peer-to-Peer Electronic Cash System and would like to make sense of the following bit from the "6. Incentive" section:

"Once a predetermined number of coins have entered circulation, the incentive can transition entirely to transaction fees and be completely inflation free"

First of all, by "Incentive" are we talking about the incentive for people to utilize their computing power to participate in the "proof-of-work" system? So currently, the incentive is that by participating, you can "mine" actual bitcoin. But when the bitcoin is all mined...what is meant by transaction fee? The person using their computer receives payment that comes from what?

I'm not great when it comes to computer science so pardon my primitive articulation of some of these concepts.