The Extent—Help | ArcGIS Desktop documentation covers it, albeit maybe not direct enough.
Summary
An extent is a rectangle specified by providing the coordinate of the lower left corner and the coordinate of the upper right corner in map units.
The key is "in map units," which means in the units of the spatial reference for the layer. From looking at the values in your screenshot below, it is obvious the coordinates aren't geographic, so you are working with some projection.