When you are working with inventory sometimes you need to make a quick adjustment to what is on-hand in the system compared to what you see in the physical world. There are a few ways to adjust inventory for example with an inventory adjustment journal or a counting journal. But there is also a quick handy function that you can find on the on-hand lists.
Here is a quick overview
D365FO 1611 Update 5 (7.0.4475.35138)
If you are looking for an overview of AX2012 you can see this post.