Flask
In plain terms
A lightweight, flexible Python web framework.
Going deeper
Flask is a minimal 'micro' framework giving you the essentials to build web apps and APIs, leaving structure up to you. It's simple and extensible.
Under the hood
Unopinionated with extensions for added features; great for small services, while Django suits batteries-included needs.
Examples
REST APIs, microservices, prototypes, small Python web apps.
Think of it like...
Like a bare studio apartment you furnish exactly how you want.