Supabase
Supabase is a service that gives your app a database, user sign in and file storage, without you running a server.
In a simple example
Your booking app saves each booking in a Supabase table, and customers sign in with their email.
Why it matters to you
It is a quick way to give an app a memory and accounts, which a plain website does not have.
Do not confuse with
Supabase is one of several such services. The idea (a hosted database) is what matters.