SQLAlchemy is a toolkit written in Python that lets you interact with SQL databases. It is essentially the glue that will bring your Python application and data together. SQLAlchemy is also an object relational mapper (or ORM), which means it translates your data in a database into objects in Python so it is easy to work with.
SQLAlchemy is a multipurpose tool to interact with SQL databases using Python. Its most common use is to map data structures in your code with data in your database.
Understanding what an Object Relational Mapping (or ORM) is will be key before learning SQLAlchemy and using it in Python. Get a quick overview of how it's used before diving in.
This is an excellent video introduction to SQLAlchemy for people who have no exposure to the ORM. It covers basic operations and setup with accompanying slides.
This is the official documentation for SQLAlchemy. It's the most comprehensive tutorial on their platform as you'll get and it's worth spending time studying it. Go through the whole thing.
It's very common for Flask or other web frameworks to be mentioned when you're learning about SQLAlchemy. If you're unfamiliar with a Python web framework, at least understand the basics before going further.
Learning SQLAlchemy in isolation is very rare and is much better understood in the context of a web framework, like Flask. The two technologies go very well together, and it's easiest to learn them side-by-side.