Aliases are a way of shortening commands, such as "c 'cure light' who" which are not exactly easy to type quickly during combat. Aliases let you type something sure as "cure who" instead.
A note about aliases, if you don't want the alias to be triggered put a space in the entry before the command, i.e. I have an alias "curse" and there is an emote curse... curse as an alias casts a curse spell, but I am talking and want to use the social, so i type " curse mtf" into the entry to curse at Mtf.