There's high and low skill floor and skill caps.
Skill floor means how easy it is to play at a decent level. You know, not feed your butt off and go 1/9 every game.
Skill ceiling is how hard a champion is to utterly master. To know how to pull the champion's entire effect and damage out from their kit. To know everything a champion is capable of in every way. Think of Garen- he's ridiculously straightforward. There's not going to be much craziness coming out of a Garen because his kit is ridiculously simple. He can auto, he can use a speedup that silences and can act as an aa reset, he can spin around, he can reduce damage, he can ult either for execution magic or true damage.
So there's some variance there and some choices to make, but it's all straightforward, it's all binary and more or less telegraphed to your opponent what he can do. He can't dash anywhere, he can't hit you from range, except with his ult and he doesn't use his ult until you're low for the execute damage. All he does is runs at you and hits you.
Skill ceiling is Azir or Yas or Zed or Syndra or TF or LB. Some are very hard because of mechanics, some are hard because of decision making, map awareness, and spacing within fights. Some are both.
High skill ceilings can be more or less apparent depending on the champion. Easyhoon's competitive Azir probably doesn't look as flashy as Arkadata's soloq Yasuo, but is still insanely hard to reproduce. Even the flashy skill ceilings can be hard to notice with close attention or analysis. I fell in love with Arkadata's Yasuohere and here when I saw a montage of it. He does things on that champ most people wouldn't realize exist. Just see if you can catch everything he does and why. He coined and afaik created his "Airblade" combo. Try outputting the damage a great Azir will do though, and you'll fall way short. The positioning and spacing someone needs for that is insane- the ability to stay out of damage, while still being able to deal damage yourself. Or check out any of the shit Faker does he'd never be able to do on something like Garen or Annie. Syndra, Riven, and of course Zed. I'm somewhat conflating #LCSBigPlays with high skillcaps... but these are things you can't do on many or any low skillcap champions.
Things like tf are hard (and don't make for highlight reels as much, except for some Misaya stuff) because they require constant and complete awareness of what's going on around you. For tf, you have to always be aware of your solo lanes, and having ideas as to where the enemy jungler is. You have to be ready as soon as a gank or pick opportunity is ready and have solid teamwork, and during fights you have to be able to weave in and out effectively. He probably has one of, if not the most beautiful kit in League of Legends, and the real power doesn't appear until high ranks. Like you honestly should never play him outside of at least diamond if you're looking to win.