Unfortunately, there is no comprehensive guide to insets.
Let me try to explain how insets work.
This is a single text field that may be empty, one integral number, or four space-separated integral numbers. E.g. "", "10", and "10 20 30 40" are valid values.
If the field is empty or its value is not one of the two other valid types, the label will have default insets. A labels default insets a 2 space units on each side.
If the field has a single integral number, the label will have a corresponding inset on each side. E.g. a value of "10" means an inset of 10 space units on each side.
If the field has four space-separated integral numbers, each number specifies the inset for one side in the order top, left, bottom, and right. E.g. a value of "10 20 30 40" means top inset 10, left inset 20, bottom inset 30, and right inset 40.
The quotes above around example values above are not part of the actual values and only serve to mark the enclosed text as example value. The quotes may not be entered into the Insets field.
Group node insets
Some group node styles support insets as well. Those that do provide four separate properties "Top Group Inset", "Left Group Inset", "Bottom Group Inset", and "Right Group Inset". The value for each of the corresponding text fields has to be a single decimal number.
Group node insets only have an effect, if the corresponding group node has at least one child node in the current view. (A closed group node never has children in the current view!)
Finally, I would like to clarify that there is no "Insets" property for shape nodes. However, when you select a node, the properties view in yEd's lower right corner will show a section "Label". The properties in this section are the properties of the node's first label. (This is the case for all node styles, not just shape nodes.)