The process of adding, mixing or injecting air or oxygen into the cooled wort immediately before or after the yeast has been pitched. When aerating your wort, it is critical to keep its temperature under 80 F in order to avoid hot side aeration which will oxidize the wort and potentially cause off flavors in your finished beer.
There are several different ways to aerate your wort. The typical methods include injection and agitation.
The image below is a carboy filled with wort that is being aerated.