This video explains the fundamental principles of indexing table columns in a database to speed up queries. It illustrates the difference between clustered indexes and non-clustered indexes, which are also known as secondary keys. It explains that the primary key of a table is normally the one and only clustered index, because this defines the sort order of the table, and that a table can have multiple non-clustered indexes. The video also mentions that database indexes are normally implemented behind the scenes as binary tree data structures, because these allow for fast searching.