Traffic areas affect how the pathfinding algorithm works. Basically, whenever a creature tries to get somewhere, it seeks through the passable tiles adjacent to it, the passable tiles adjacent to those, the passable tiles adjacent to those, and so on, until it has sought through every tile accessible to it. Then, it adds up the number of tiles between it and its target along every possible route and sends the dwarf along the one with the shortest path. Traffic designations affect how long a path is considered to be when the algorithm is deciding which is shortest. A 10-tile path that's set as normal traffic will have a "length value" of 20 under default settings, for instance, so a 15-tile path that's set as high traffic (with its "length value" of 15) would be taken preferentially. This doesn't affect the speed at which dwarves move along the pathways but rather means that given 2 equal-length paths to one point dwarves will usually take the higher-traffic one.
The short of it is, high traffic areas encourage dwarves to walk along paths in those areas, while low traffic areas encourage dwarves to avoid going through those areas.
To remove a high, low, or restricted traffic area, just paint a normal-traffic area over it. By default, everywhere is set as a normal traffic zone, and things can only have one traffic level set at once. Also, only dwarves respect traffic designations.