Many recipes call for the cocktail glass to be rimmed. This can be anything like sugar, salt or cocoa powder. This is used to enhance the taste of the cocktail or sometimes just for decoration.
First, you moisten the rim of the glass with a lemon or lime. I like to use an orange because it is a little stickier than the others. Ideally you rim the outside of the glass to keep your ingredient out of the drink, but this takes a little practice and patience.
Then you roll the glass in a plate filled with the sugar, salt or whatever until the rim is completely covered. Ideally, you want to do this about 15 minutes before you fill the glass. This gives time for the rim to dry and will keep your sugar, salt or whatever from falling off.