Database indices work like book indices: Without them, you'd have to flip through every page to find occurrences of particular topics.
In both cases, indices are only useful if they make it easier to find useful information. You wouldn't have a book indexed with every word in the book, because no one needs to find every occurrence of the definitives and conjunctions. For the same reason, it's important to ask yourself how you'll need to access information on a given table. Will you need to look up users by email? Username? Both?
Answering this question up front will keep your table efficient later. Without the right database indices, you'll end up asking the database to perform a full-table scan each time you use a method like findbyemail.
If you do need to retroactively add an index to a table column, you can:
rails g migration addindextousersemail
Write the migration:
def change add_index :users, :email, unique: true end