The martial/caster disparity is the (IMO, proven and obvious) idea that martial characters lack gameplay options compared to their caster counterparts, and that this problem only ever gets worse with level.
Also, “martial” in this case specifically refers to Fighters, Barbarians, and Rogues.
IMO, it does exist, but it’s not as “end of the world”-bad as some people make it out to be. Basically, rogues are fine because they get a crap-ton of skills that can be put to good use as long as the rogue player makes their character with even a little long-term thought. Rogues that have problems tend to focus in things like stealth, and other physical skills that casters can use spells to imitate or replace. Rogues that pick up and spend expertise in one or two soft skills (some kind of knowledge skill, insight, investigation, etc…) will never find themselves with nothing to do and will always have a niche where they can make the full casters go “holy shit!” from time to time.
Fighters and Barbarians actually have problems because they seem to have been made more with dungeoncrawling in mind, to the detrement of anything non-dungeon related. They generally lack useful soft skills, and don’t stack stats that will make using them useful because they generally don’t have ways to make a high int or wisdom terribly useful.
Fighters and Barbarians compound the skill problem by not gaining useful/impactful abilities in T3 or T4. When full casters are busy choosing and enjoying the most powerful spells in the game, fighters get another use of indomitable (which never, ever fucking works, IME), a second action surge per short rest WAY too late for it to really matter, and a 4th attack they will probably never, ever actually get because it’s at 20th level for some stupid, fucking reason (as opposed to level 17 where ALL casters, even the half-casters, get their 4th cantrip damage die).
Barbarians get even less than fighters due to most of their class budget being tied up in a massive passive ability: Brutal Critical. So all they ever get to do is crit-fish, which they’ve all been doing since level 1 anyway.
The disparity is choice and impact. Because of their lack of choices, it can seem difficult to have an impact on the game, mechanically. A good DM can make up for this in a variety of ways, but when you’re just looking at the rules or white-rooming a character, the problem does tend to become a bit obvious…if overblown.
Generally, the fix is simply to give fighters and barbarians more class abilities that involve getting to make interesting choices during play.