“The first subscription service for Mac apps”, as you can read on their website. And that’s exactly what Setapp is: a subscription service for macOS applications.
For the $9.99 monthly fee, you can install on your Mac any of the Apps they have a deal with.
On the app´s early days, there weren’t many interesting apps that made Setapp worth buying. But that has totally changed. Nowadays you will have apps like CleanMyMac, Forklift, Permute, Screens, Sip or Ulysses. These apps separately are more expensive than Setapp a year itself.
If you want you can try Setapp 7 days for free and decide by yourself wether it suits your needs or not.