Software as a Service means you do not install or run anything yourself. You log in to an application over the internet, and the provider looks after the servers, security updates and new features, usually in exchange for a regular subscription.
It lowers the effort of getting started and keeps you on current versions. The trade-offs are less control over the product and its direction, and dependence on the provider for availability and data handling. Check how you can export your data, because that decides how easily you could move to something else.