First, it's only $8 for the Numenera Player's Guide if you want to check it out, but don't want to spend too much.

Second, it's about the stats you need. Even in 5e, the simplest of the recent (last 20 years) editions of D&D, if you need to have monster stats, you need AC, HP, attack bonus, damage, abilities need DCs, etc. If you are experienced enough, you can do a pretty good job of making those up on the spot, and in 5e it's actually not that hard.

Numenera makes it even easier than that. Pick a level for whatever it is the heroes are fighting, and all the other information you need can be derived from that. HP is lvlx3, damage = level, difficulty of doing just about anything to it is =level. It's just very simple, and streamlined. Traps use the same system. Magic items, npcs, and anything you can think of basically follows this same basic format.

Give something a level from 1 to 10, and you are done.

Like I said though, comparing it to 5e isn't saying a whole lot, it just barely makes it a bit simpler. Coming from 3.5 or 4e, and holy crap is the difference huge. To convert an adventure from another system and make it reasonably difficult for the parties level in 3.5 could take a week or more of work. In Numenera you could do it on the fly. Like, just bring the book with you and convert as you go, taking only as long as it takes you to read it.

It's also simpler in the sense that there are only so many different ways to effect a roll. There is skills, assets, and effort. Skills and assets can effect the difficulty by 2 each at the most, and effort is limited by level. There aren't hundreds of modifiers to think about, and not much math to do. It's also just the one die. You roll a d20, and if you meet or exceed the difficulty level x3, you succeed.

Also, weapons use static numbers instead of rolled dice, a minor simplification.

Being super simple is only one of it's selling points though, and I highly recommend the system. Don't feel constrained to the Numenera world either. The sister system, The Strange, expands it to just about every genre, and they are working on a setting-less version of the system too (not sure when it's coming out though).