I don't understand tiers and my knowledge of Psionics begins in 1e and ends in 2e.
Tiers are how flexable + powerful a character is.
They range from tier 5 (Can do one thing, period, and other stuff out there can often do it better), tier 4 (Good in one area, plus has some stuff outside it, or ok in lots of areas), tier 3 (Good in multiple areas), tier 2 (Good in a variety of areas, though sometimes requiring some work), and finally tier 1 (doesn't take much work to be good in pretty much any situation).
Like, Wizards are tier 1 since each day, they can customize their spell list to their likely tasks. Need to smash? Summons. Need to sneak? Invisibility, knock-spell, silence. Need information? Scrying spells.
Sorcerers are tier 2 since they can't adjust their spell list, they only learn spells when they level, though careful selection can have them ready for a wide range of stuff.
Bard's tier 3 since it can buff reasonably well, fight ok, do a pretty good job at having skills, and the limited casting. A number of 'has some spellcasting, but limited,' types fall in here.
Barbarians are tier 4 since they can smash stuff *really* well, plus good skills and their rage powers and abilities give them some usefulness outside of combat. Rangers and Paladins, likewise, they can fight and have some supplementary powers.
Rogues are tier 5. They can skill, but nothing a Bard can't do (or a wizard can't replace with proper spells). They can fight about as well as a bard, but don't have the bards spell list. Fighters are tier 5 since they can fight and that's it, their skills are worse than barb, paladin, or ranger.
Warriors and Experts are tier 6. Warriors only fight, but worse than fighters. Experts only skill, but are otherwise worse than rogues. Truenamer are tier 6 despite intending to be a serious class, because Truenamers are just that poorly designed.
Psionics is entirely different in 3E, and Tiers are also 3E-only due to how poorly balanced it is. Basically spellcasting is objectively better than all other classes like in no other edition before or after.
One could do tiers for prior editions, but it's rarely worth bothering. I mean, *of course* Paladin is better than fighter, it's a fighter + other stuff. And Rogues are just kinda weak, but necessary because there's not really alternatives to getting some of their skills.
And 'wizards start out weaker but end up stronger,' is a well-regarded fact, but it takes longer for crossover to occur, and monsters have fewer HP and 'win the battle spells' rarer and harder to use, so the fighter has a bigger effectiveness sweet spot.
