Just like the index in a book a database index will help to find specific things.  Unlike most books a table can have more than one index.  The idea is to create an index on a field if it regularly gets searched.  So in a table about people the surname would definitely be indexed and maybe others.  Indexing a field involves writing extra data to a table so it takes space and processor time to create.

Without indexes a database management system would need to look at every record in the table to find matching data.  Indexes are sorted so it can quickly jump to the matching data.  With the tables you are creating here there is not enough data for indexes to make a difference.  When you do real sites index any field which will be regularly searched in queries.