Swimbladder is not simply a phrase. It is an organ in most fish species, that helps control their bouyancy. Like most organs it can be injured, become diseased, have genetic weaknesses or be compromised in function by other causes.
Constipation, often caused by improper feeding, or sometimes by conformation of the fish, can put pressure on the bladder and show up as one or more issues with swimming, and this is not uncommon in gold fish, particularly the short bodied fancy ones.
Swim bladder is blamed for many things, especially in gold fish, and I do agree it is not always the problem, but it's a real organ, not a phrase.
Cooked, skinned peas are a common remedy used by many fish keepers for constipation, in discus and other species besides goldfish.
I can't comment on how effective they are, or even if they are the best remedy, but it's certainly one often seen mentioned.
Feeding high quality food and trying to incorporate as much of the kind of food the fish would eat in nature is, IMO, always better than feeding an entirely man made diet. Some goldfish, due to their conformation, which is the result of selective breeding by humans, don't have a real stomach, so in these fish, feeding is even more important, as food goes directly to the intestine, without predigestion in the stomach.
Some fish seem to have more issues related to swim bladders than others, but diagnosis in fish is often difficult. We have few vets who treat fish we can access, many symptoms with many possible causes, and few really effective remedies for serious afflictions. Observation and keeping records of what you did with the tank or fish in question before symptoms showed up might be useful, but many fish keepers are not great record keepers. So we do the best we can based on the information we are given or have and in many cases, fish get better without help or in spite of anything we do.
And sometimes they don't get better, which is sad, but it's life too.
