Tar seals timber by filling surface texture and forming a continuous water-shedding coating. It slows moisture entry, but it is usually a breathable, maintainable seal rather than a permanent plastic barrier.
Timber is full of grain lines, pores, small checks, and cut ends. Tar settles into that uneven surface and creates a darker, less absorbent layer. This layer reduces how quickly rain can enter the wood and how easily the surface can be attacked by weather.
The seal is not the same as wrapping timber in plastic. Traditional tar coatings can remain slightly flexible and may allow some moisture movement over time. That can be useful on old buildings and outdoor structures where trapped moisture can be a bigger problem than a coating that slowly breathes and can be renewed.
A good seal depends on continuity. End grain, joints, cracks, knots, nail holes, and old splits are weak points. If those areas are missed, water will find them. Tar does not know where the vulnerable parts are; it only protects the surfaces it actually reaches.
Timber sealing with tar is therefore a surface-management method. It slows water, darkens the wood, adds a protective film, and gives the owner something that can be inspected and refreshed as it weathers.