An indexed journal is one that was accepted by an indexing database. Each discipline has its indexing database ,for instance, psychology has the Psychinfo database where papers of psychology journals can be searched. But there are databases that include papers from all disciplines, such as Scopus and Science Citation Index.
So, your range of interest establishes where you are looking for journal articles. The Scopus is widely used, but the Science Citation Index is considered more selective, and highly valued.