Session management is a way to restore the GIMP session that was running prior to quitting the GIMP. I.e. simply quit GIMP and start GIMP again. GIMP will now along with the Window Manager try to restore the GIMP session you that was running before you left GIMP. You can disable or enable this feature with the Always Try to Restore Session checkbox.
GIMP can save its window positions — where, for example, the Gradient Selection dialog is displayed on your display. If you use the Save Window Position on Exit option then GIMP will save the positions of open dialogs. It can be useful to arrange your GIMP desktop with this option enabled and then restart GIMP and disable the option. You will thereby have a consistent desktop each time you start GIMP.
If you have a graphics tablet and use it with The GIMP, you will have several devices that you can preconfigure in the Device Status dialog. However, if you use the Save Device Status on Exit option, the preconfiguration will be overwritten and the last state before exit will be the new configuration.