Traffic locks are editing restrictions that Waze automatically places on busy road segments. The busier the road, the more locked down the segment becomes. This feature is fairly new to Waze, and only applies to some countries.
Traffic locks are rated rank 1 through 6, corresponding to the editor ranking system. Traffic lock of 6 is currently the max but each country is in negotiation with waze on what will be the max... 1 "traffic cone" is the lowest ranked editor, and 6 traffic cones is the most senior. Rank 6 is generally reserved for Waze Local and Global Champs.
This correspondence means that, for example, a busy road traffic locked to rank 4 will only be editable by editors with ranks 4, 5, or 6 traffic cones. Editors ranked 1, 2, or 3 will not be able to edit those segments.
For more details, see Editing permissions. Also note that there are other types of editing restrictions besides Traffic locks. Those other restrictions, and how they interact with Traffic locks, are all explained on the Editing permissions page.