Adjuncts are sources of fermentable sugars that aren't from a malted source.   Examples are rice, corn syrup, sucrose and molasses.   Sometimes the sugars are readily available to the yeast in fermentation - like in sucrose - or need to be added to the mash so that the malted grains enzymes can be set to work on them - like torrified wheat.

Usually the adjunct is used to make a beer cheaper to produce and has little flavour of its own.   Sometimes the adjunct provides a flavour or quality to the finished product that makes it distinctive.   Many cheaper lagers use corn syrup to pad out the malt, giving more units of alcohol per cost price.   One particularly famous American lager started using rice as a cheap source of carbohydrate to reproduce European pilseners at a time when European malt was very expensive to import.   Now the grade of rice it uses is not cheap at all and it gives the beer its distinctively unchallenging flavour.

Belgians brewers use adjuncts to give them a wide variety of weird and wonderful flavours such as candy sugar to give a vanilla, candy floss sweetness.   Honey, maple syrup and molasses can give a distinctive taste with only a little sweetness.