Object-relational mapping is a technique that lets you query and manipulate data from a database using an object-oriented paradigm. An ORM library is a completely ordinary library written in your language of choice that encapsulates the code needed to manipulate the data, so you don't have to use SQL, you can interact directly with an object in the same language you're using.
reference - stack overflow
If you are a node.js beginner, you may not be familiar with SQL database since we mostly use document database such as mongodb with node. However, it is not too complicated to use SQL database with node.js. I used postgreSQL for this repo. There are many orms available for node.js such as prisma, sequalize, typeorm. For this repo, I used typeorm.
TypeORM is a TypeScript ORM (object-relational mapper) library that makes it easy to link your TypeScript / JavaScript (ES5, ES6, ES7, ES8) application up to a relational database.