Userscripts are basically community written add-ons that adjust some functionality of WaniKani. They let us do things like adjust what information we see on the dashboard, reorder lessons within the level (do with caution), get a list of words that you miss often (leeches) and study them, show extra information in lessons…lots of great stuff created by the community. There’s a master list of them (and other related apps) here - The New And Improved List Of API and Third Party Apps
You have to install a browser extension like TamperMonkey to use a userscript - some people don’t like to do that and therefore don’t use userscripts - totally up to you.
DoubleCheck is my preference for handling typos - you can find it here - Double Check Script
This guide to WaniKani is really helpful for understanding how the system works more in depth and is recommended reading for getting started and wanting to use your time wisely. Also, explains scripts and other stuff way more eloquently than I can.