Full explanation
SaaS stands for Software as a Service. Instead of installing a program on your computer, you log in through a website and use it from anywhere.
The company hosts the software, handles updates and security, and charges a monthly or yearly fee. Notion, Spotify, and Canva are SaaS products.
For a founder, SaaS is attractive because it brings in recurring revenue. Each customer keeps paying as long as they get value.
Concrete example
You build a tool to help restaurants manage reservations. Restaurants pay 29€ per month to use it. That is a SaaS.