Slots are the invisible glue that bind Widgets together. In Slate they are a lot more explicit in that you must first create a Slot then
choose what control to place inside of it. In UMG, however, you have Panel Widgets that automatically use the right kind of Slot when Child Widgets are added to them.
Moreover, each Slot is different. For example, if you were to place a control on a Grid, you would expect to be able to set things like Row and Column. But these properties have no business being on a Widget that was placed on a Canvas. That is where Slots come in. A Canvas Slot understands how to layout content absolutely and through Anchors, while a Grid Slot only understands Rows and Columns.