Displays tell Views where the output should go. By adding a display to a View, you can have your view appear as a page, or as a block, or even as an attachment to a different display on the view.
Each display can have its own settings, but when it's created, a display will take all of its basic settings from the default display which all Views must have. For most settings, there is an override button that will override that single setting for the current display. Overridden settings will have a mark in the summary for that display. All 'default display settings' are shown in the other displays 'italic'. When you override a setting, than it is shown 'normal'.
Please keep in mind that when you are editing a setting on a display that is not overridden, then by default you are editing that for all displays.
For fields, arguments, sorts, filters and relationships, you can only override all of them or none of them. To do this, click on the header for the filters or the rearrange button. Once you override, the display will then have its own copies of the fields/filters/etc and changes to the defaults will not be reflected on your display.