What's the ItemPointer's meaning here?

/* Typedef for callback function for table_index_build_scan */
typedef void (*IndexBuildCallback) (Relation index,
                                    ItemPointer tid,
                                    Datum *values,
                                    bool *isnull,
                                    bool tupleIsAlive,
                                    void *state);
When we build an index on an existed heap table, so pg will read the tuple one by one and give the tuple’s hepatid as
thisItemPointer, is that right?