To liaise, or to make a liaison, is often required in baking when making custards and also when thickening some kinds of sauces.

The goal is to temper the egg yolk, to bring it up to the temperature of the hot liquid, without making mini scrambled eggs.  The technique of  adding hot liquid to the egg yolk separate from the main pot helps you out with two things:  1) you will be able to control the process, therefore if it does break, making little granule-looking pieces of egg, you can start over; and 2) the egg yolk will be closer to the temperature of the entire hot liquid therefore enabling a consistent mixing and thickening of the sauce or custard.

First, whisk the egg yolk in a bowl.  Then collect some of the hot liquid in a measuring cup (with a measuring cup's spout it will be easier to pour).  In a small stream pour the hot liquid into the egg yolk very slowly.  The entire time you should be whisking.

After you have successfully incorporated the egg yolk and hot liquid, add the yolk/liquid mixture to the entire sauce or custard.