Software as a Service - using software applications over the internet without installing them on your computer. Like using Netflix instead of buying DVDs - you access the service, not own the software.
Gmail, Salesforce, and Slack are SaaS applications you access through a web browser without installing anything.
SaaS is a cloud delivery model (not a single cloud provider service). AWS, Azure, GCP, and OCI primarily provide IaaS/PaaS building blocks that SaaS vendors use to host and deliver their applications. Each provider also offers some SaaS products (e.g., email, collaboration, analytics), but there is no one-to-one 'SaaS service' equivalent across providers.