The detail block shows the values of a model's properties. You can select which properties appear in the block and the order they appear. The detail block is similar to the form block but is read-only. You can add some standard text, links, and dividers to the block. You can also group the data shown in groups.
If you had a page with a list block and added an action to open a page with a detail block, the block would show the details of the list item record.