Tempering eggs is a critical step in making a traditional custard. It slowly brings the temperature of the eggs up as they’re cooked. Heat them too quickly and you’ll end up with scrambled eggs.
How it’s done is you heat a milk mixture just until it starts steaming and tiny bubbles form at the edges, and then add it in a slow steady stream to the eggs while whisking constantly. After they are combined, you can return the mixture to the stove and continue cooking until thickened. Some eggs may still form lumps as they cook, so straining the mixture before freezing will leave your custard silky smooth.