Aliases allow users to find games under different names or acronyms that they are commonly referred to as.
For example, Star Wars: The Old Republic has the following aliases:
Star Wars MMO
You can see for yourself how an alias works by typing one of the above nicknames for The Old Republic in the search window in the upper right corner of your screen. Try putting in Star Wars MMO...as you can see, it immediately finds the correct game.
Since aliases affect Giant Bomb's search engine a great deal, please only request to add or remove an Alias if it is critical for the page you're suggesting it for. Aliases need to be used sparingly so the search engine doesn't get completely filled with bad results - this is why Alias is and will remain a staff-only field. I'll use The Old Republic again as a good example. Since this title is new, many people may not know it's called The Old Republic, but they may know that there is a Star Wars MMO in the works and in this case, the alias is a success and guides them to the correct place.