To KU or not to KU (Kindle Unlimited)
The advantages are 1) readers can read your books without paying any more for it than their monthly subscription fee, and 2) you have 5 free days every three months. There are also Kindle Countdown deals, which I've never used but many authors do.
The disadvantages are 1) your book is exclusive for 90 days with Amazon, and 2) you are now more dependent on Amazon for your business.
For anyone who doesn't have many books, doesn't have money for advertising, writes in popular genres in KU...it can be a big benefit. Other authors would rather be "wide" which means not in KU and able to list their books everywhere else AND Amazon (but not in their Kindle Unlimited program).
It's 90 days either way, so if you decide to try it...you're not bound to KU for more than those 90 days. If you look around in this and other forums, you can see KU vs. wide is a major discussion topic. Best of luck with your book and future sales!
ETA: I have some books in KU and some wide, so I base the choice on genre, whether I want to sell that book on my own website, if it's part of a series that does well in KU, if it's free first in series, etc.