Locked segments 
To protect the map from unwanted errors, some segments are secured with a lock. Important main roads will show higher locks than small side streets. Locked roads can only be changed by editors with a rank the same or higher than the lock level. Even if you succeed to edit such a segment, you won't be able to save your changes. Also, you will not be able to connect other roads to locked roads. If you try to do that, it will result in disconnected segments, like described in the previous paragraph.