An override temporarily changes the end action of the targeted object. E.g.:

You got a timeline A with an end action to go to timeline B.
On menu M you got two buttons:
Button X linked to timeline A. The override setting on button X is "Not Set".
Button Y also linked to timeline A. The override on button Y is set to "go to menu M".

When you start timeline A with button X the DVD will play timeline A and then timeline B.
When you start timeline A with button Y the DVD will play timeline A and then go to menu M. This is because the override setting in button Y's Properties has temporarily changed the end action of timeline A.

As for your menu looping issue: If the main menu was set as "First Play" and it didn't loop the at the start of the DVD I can't explain how that could have been caused by an override setting. If on the other hand the problem occurred when the main menu was selected via the "previous" button on your submenu (the one with the override setting) then it could be quite possible that the override cancelled out the end action (loop) of the main menu.