Basically Infostore refers to the metadata about Objects and in BO terms – infoobject. An infoobject is basically an entity in SAP business Object universe. It can be a report or a user or an event as well.
For E.g.: if we are speaking about an employee so an employee has a metadata like Name, Department, Designation, Salary and others. Infostore lists out this metadata when the context in Employee. So when you execute a GET request for getting infostore it returned you the metadata for an info object.