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 service model (not a single provider service). AWS, Azure, Google Cloud, and OCI mainly provide IaaS/PaaS building blocks that SaaS vendors use to host and deliver their applications. Each cloud also offers some SaaS products (for example, email or collaboration tools), but there is no one-to-one “SaaS service” equivalent across these providers.