I believe that was a strike at their centrifuges. They are needed to enrich uranium to powerplant grade or weapons grade.

Iran had agreed to stop using them, as part of the treaty trump tore up. Afterwards, the lack of any detectable fallout indicated that they were not in use, since no uranium was present.

Basically they lost their ability to produce more weapons grade material. It is unknown how much (if any) was previously made, and might be stockpiled elsewhere.

It also shows that Iran seems to have remained in compliance, despite trump pulling out of the deal.