Dextrose: AKA glucose, corn sugar is a monosaccharide the easiest form of sugar for yeast to consume. Not to be confused with Dextrins, which are not fermentable.
1KG (2.2LB) of corn sugar added to 20 liters (5.28gal) of wort will raise the OG .019 points.
Many recipes make this addition in the boil though usually not more than 1lb per 5 gallons. This gives the gravity a nice boost for some clean ABV increase with a slight dry character. But is usually balanced with the malt and goes unnoticed.
If yeast is given too much glucose it will basically forget how to consume the larger sugars unless the pitch is very large. What happens is the yeast will feed and reproduce from the glucose first and literally lose the pathways to more complex sugars from cell division scars and then be unable to consume very much of the complex sugars. Also the ABV raises faster than the cell walls can adjust to tolerate. Leaving you with an under attenuated sweet beer.
If you plan on adding more than 1lb into 5 gallons I recommend 1lb into the boil, and the second 1.2lb with nutrients as a feeding when fermentation starts to slow.