Stupid little plastic figurine characters that are normally about £8-20. They have chips in them that alter the gameplay in Nintendo games when you he’d on the NFC point.
Usually it’s just costumes, stickers, etc. Cosmetic stuff. Some do really good things like unlock levels, give you hearts or secrets or have shortcuts to DLC. They work in multiple games too.
In Smash Wii U, you could train an amiibo to mimic your abilities, meaning if you wanted to hard s harder fight, you could effectively battle yourself. I tried it a few times and it was pretty good. Not sure if they’ll have that feature this time or not, but worth buying one to see if it gives you ‘the bug’ or not. Pray it doesn’t, as there are about 100 of the bastards