I'm playing this game way too late, but did pick it up for a few pounds. Super Mario Maker has a level creator and you upload them to the Nintendo Network. Nintendo are going to be shutting down the servers next year, so it's got limited shelf life now.
Not all the tools are available from the start, but delivered over many days of playing. There's a great amount of objects to create your levels. You place blocks, items, enemies, warp pipes, vines, springs, keys, red coins that grant you with a key to unlock a door, extra sound effects, visual effects. You can shake selected objects to modify them. Apparently checkpoints were patched in later which could be why they seemed like a rare occurrence. You have the option for normal levels, or autoscroll. You can style the graphics with most 2D Mario games: original, Mario 3, Super Mario World, New Super Mario Bros.
You are allowed to create levels which would never exist in the actual games. For example, the Ghost house levels never appeared in the original game but you can use it with the original game theme, so Nintendo has created brand new sprites to support this.
There's a special mushroom, which gives you a character skin so you can play as all kinds of characters: Pikachu, Excitebike, Link and loads more.
The creator has to beat the level once for it to be valid, but that doesn't stop them making something almost impossible, or requiring you to do some advanced technique, or find some fairly obscure route through the level when the level implied a different path e.g. have a hard to reach warp pipe as the actual route forward).
You can play single levels, or randomly chosen levels in a set. I didn't really get the difference between 10 and 100 challenge because it's still a randomly selected batch of levels. You have difficulties within, and with a higher difficulty you have a longer set and they are generally way more difficult. Unless you are extremely wasteful of lives then you are going to complete the Easy and Normal modes. You can save and quit so its not even an endurance challenge. If a level is too hard, you can skip and swap to a different one.
You can see where people have died with an X speech bubble. I think this used to have a Miiverse post, but Miiverse closed down years ago.
The levels can be put into certain categories. There's well-made standard levels, ones which are short and have maybe 1 challenge, ones where you just run forward, ones where you just sit and watch, ones that are just trolling, and ones that are pure carnage. I was actually surprised that the quality was higher than I expected, but there were still too many bad ones, and too many of the ones you just watched. I think those ones are clever and must take a lot of trial and error to put together, but what is the point?