What is SAP Persistence Service? One of the simplest definition is that it forms a bridge between your relational database and ABAP Objects.
The Persistence Service lets the ABAP programmer work with relational database data in an object-oriented way.
If you have any experiences in other programming languages, like Java or C#, then maybe you have heard about ORM (Object-Relational Mapping) frameworks. These ORM tools enable you to construct classes with attributes, and then the framework will automatically generate the corresponding database tables in your database, and represent the data like objects.