Flow Builder is the declarative interface used to build individual flows.
In Salesforce, a flow is a tool that automates complex business processes.
A Flow is an automation configuration saved in Salesforce with the structure of a flowchart.
Simply put, it collects data and then does something with that data.
Flow Builder can be used to build code-like logic without using a programming language.
E.g. you need to perform mass updates across multiple unrelated records.