Dry-ageing beef means that once the animal is slaughtered and butchered, portions of the carcass are allowed to rest in very carefully controlled conditions for a period of time - often several weeks, and sometimes up to a couple of months. When we create such conditions, we allowed enzymes to do their work and we end up with a complexity of flavour of a dry-aged piece of meat. The dry ageing process comes at a cost; as much as 30% of the weight of the beef is lost as moisture is evaporated and the flavour concentrates, also the surface of the meat begins to break down and this needs to be cut away before sale and use.
The enzymes that occur naturally during this process begin to break down the meat's proteins, fats, and glycogen ( a carbohydrate) into amino acids, fatty acids, and sugars. One amino acid generated by dry ageing (the most important and flavourful one) is glutamate, which is part of MSG.
Meat begins at about 75% water, after ageing it may go down to somewhere around 70%. This might not sound like much of a change, but what that means is that the flavours become more concentrated, and the tissue itself becomes more concentrated. Dry-aged meat is still juicy when you cook it, but the juices are even more delicious than usual.