Tempering is a method of mixing eggs with a hot liquid without having them clump and cook too soon. The trick is getting eggs used to the heat so they don't cook before mixing them into the main batch.
If eggs are dumped into a hot liquid they turn into scrambled eggs, ala Egg Drop Soup. This is fine if you are making Egg Drop Soup, but is nasty for custard, where a smooth consistency is what you want.
To temper eggs, make sure they are well beaten. Whisk some of the hot liquid into the egg mixture, a little at a time. Once you have about a third hot liquid to your eggs (1 cup total, 2/3 C eggs, 1/3 C hot liquid) you can go the other way, whisking the now tempered egg mixture back into the remaining hot liquid.