Segment locks, also known as manual locks, are editing restrictions on certain road segments. This type of lock is manually applied by a higher-ranked editor to a particular segment of road that needs to be protected from accidental change or vandalism. The more important the road, the more likely it is to be locked.
Other than the fact that they are manually applied by an editor, they function similarly to automatic traffic locks. For an overall description of the various types of editing restrictions, see the Editing permissions page.
In brief, there are six locking ranks. An unlocked segment has a lock rank of 1. These six locking ranks correspond to the 6 editor ranks of "1 traffic cone" to "6 traffic cones." To edit a segment, its lock rank must be equal to or below your own editing rank. The segment must also belong to one of your editable areas. See Editing permissions for dull details of all the restrictions and how they interact.