Espresso: what is crema?

Found this great explanation for that mysterious and wonderful froth yclept crema:

The most common reason that most people don’t get a crema on their espresso
at home is that they are using an incorrect grind of coffee. The grind of the
coffee is the most important thing in espresso as it determines the resistance
to the machine’s pump which in turn affects the brew time/contact time of the
coffee. The finer the grind, the slower the brew, the more flavour we extract.
Most, if not all, preground coffee bought in a supermarket or store is too
coarse, no matter what the label tells you. The correct grind should allow 2oz
of liquid through a bed of 14-18g of coffee in around 25 seconds (quite
slow).The other reason is that coffee stales very quickly once ground and not
only are aromatics lost but much of the CO2, so necessary for crema, is freed in
the process of grinding.