Blanching is to briefly and partially cook food, usually vegetables or fruit, in boiling liquid, usually water but oil is sometimes used.
The technique is useful for preparing vegetables for the freezer. In which case, once the food has been briefly cooked it’s usual to arrest the cooking process by plunging the fruit or vegetable into iced water. (This is called ‘shocking’ or ‘refreshing’.) The blanching technique is also used when preparing vegetables ahead of time. The blanched vegetables, being part-cooked, are quicker to fully cook later at the required time.
Usually when making roast potatoes, the spuds are parboiled or blanched before putting them in the roasting tin to roast in oil or lard.