How–and why–do I temper eggs?
When you need to add a hot mixture to raw eggs (or raw eggs to a hot mixture), it is usually important to not let that hot mixture start cooking the eggs, or you'll end up with lumps. For that reason, you "temper" eggs.
The process of tempering is to slowing bring your two items closer in temperature to each other. To temper eggs with a hot batter, I will take a little of the batter and slowly whisk it into the eggs. I will continue adding the batter, a little at a time, taking care to incorporate the batter fully and smoothly with each addition. In that way, my eggs become warmer without coagulating from the heat.
See the recipe for Butterscotch Pots de Crème as an example of a recipe where you temper the eggs.