u/675longtail on Reddit summarized it:
It includes a Starship development timeline, starting with an orbital flight attempt in Q3 2023 but moving on to detail:
Q1 2024: First Starship launch with payload
Q3 2024: Successful recovery of the "Starship system"
Q3 2025: On-Orbit Propellant Storage System Preliminary Design Review
Q2 2026: Starship On-Orbit Servicing/Recovery/Docking Concept Review
Q2 2027: Crew Starship Ascent, Entry, and Landing Concept Review
Q4 2028: Starship LEO crewed space station Preliminary Design Review
And lots of other tasks too. (For example, I have no idea what "a human health countermeasure tech demonstration" might be.)
A Space Act agreement doesn't involve payments between NASA and the company. But it does involve NASA and the company agreeing to do specified space-related things, where NASA will
Provide access to requested NASA technical data, lessons learned, expertise support, services, facilities, equipment, and NASA-developed technologies, on a non-interference basis as resources permit.
and SpaceX will "Provide NASA with data regarding its progress towards the milestones", meet periodically, and provide equipment if required under agreed Technical Implementation Plans.
The Ars Technica story was edited based on an FAA ... tweet.