A "cresty" horse is one that has a thick, arched neck. Crestiness is normal in stallions - heavy muscle on the top of the neck is a secondary sexual characteristic in horses - and in some geldings that are cut late (after puberty). Some breeds - draft and cob breeds - also have a certain amount of heavy neck muscling.
However, in normal riding mares and geldings, the "crest" is usually not muscle - it's fat. When a horse starts depositing fat above the spine on the neck and loins, it's a sign that the animal is crossing the line from overweight into obese.
A cresty horse, thus, is most often a very fat one that needs a diet and exercise plan, but can also be a breeding stallion.
This is a Hanoverian stallion with a solid, muscular crest. He is in very good condition (note the brand on his hindquarters that identifies him as an approved breeding stallion). Image source: Kersti Nebelsiek via Wikimedia commons.
This pony, on the other hand, is obese. Note that the "crease" of the crest is further down than on the healthy stallion - and he also has obvious body fat on his midsection and hind quarters. Diet and exercise stat! Image source: Dezidor via Wikimedia Commons.