Malt extract is, as the word suggests, an extract of malt - so the mashing has already happened. Therefore, if you brew beer with malt extract, you no longer have to do the mashing yourself, simply boil the wort. This is a way of brewing that is employed by many home brewers in America (the so-called mini-mash method).
You do have to take into account the fact that one kilo of barley malt does not contain as much sugar as a kilo of malt extract: 1 kg malt extract powder = 1.60 kg malt!
Proteinfree, EBC 18.