Organising products in the category is as simple as dragging and dropping them from one position to another. The positions of other products dynamically shift as you move the product around the grid. When you let go of the mouse, the product will have shifted position.
You can also move more than one product simultaneously. Just shift-click one or more products in the grid, and their headers will turn blue. Then drag-and-drop any one of the selected products around the grid. Again, the other products in the grid will shift positions around the one you are dragging. When you let go of the mouse, all the highlighted products will shift position.
Nothing is saved, until you say so!
As you drag-and-drop products around the Merchandiser window, no changes are actually being made to the category in Magento. The only time product positions are actually saved in Magento’s database is when the button is clicked.
This page relates to version 2.2.10.
The screenshots on this page are from Magento Enterprise Edition. However, functionality is identical on Magento Community Edition.