In the beer world, an adjunct is a source of sugar used in the brewing process that isn’t malted barley.
This includes corn, rice, wheat, rye, oatmeal, honey, plain sugar, and a few others. Most of these have starches that need to be broken down into sugars before the yeast can make beer out of them.
And aside from adding sugar, they can increase head retention, improve mouthfeel, lighten the body, and/or provide desirable flavors. Now that’s not so bad, is it?