The mash is where the starches in the grist get converted to sugars. This is where the enzymes developed during the malting process do their work.
The basic overview: the crushed grains of the grist get mixed (or mashed) with water. The water warms the enzymes and floods them out of the grain to be able to free float around with the starches. The enzymes convert the starches to sugars. Once all the conversion is done, the liquid is separated (a process called lautering) and is
rinsed (called sparging). This liquid is known as sweet wort. It then goes on to be boiled with hops and fermented into beer.
That’s the (very) short version. Let’s expand.