Slash-and-burn agriculture is best defined as the process of

The cultivation of farming that uses slash and Burn technique to grow the next crop off season is known as fire fallow cultivation. This is done by keeping aside the slashed or sound vegetation a side. So that it could dry up in Sun and it is taken care of that is done before the rainier part of the season.

And then the rest of the area is cut down and deforestation, then burn to grow the next lot of crops. Burning them the Ashes remain in that area which will mix with the soil to make it more nutrient rich as these Ashes contain the best and the nutrient from earlier growing plants.