Sorting options determine how your cards are sorted.
While browsing your collection, you can change your Sorting setting using the first drop down menu on the right of the search bar or by clicking the column headings while in List View. You can also set the default way your cards are sorted in the Preferences section of your Settings page.
You can sort your cards by:
when you added them to your collection
their name or set name, alphabetically
their release date
their price
their power, toughness, rarity or converted mana cost (CMC)
the amount you own
Some sorting options are not available when you have particular Grouping options set.