Weld penetration is the depth to which the weld metal fuses into the parent material below the surface. A 'full penetration' weld goes all the way through the joint thickness and develops the full strength of the parent metal in tension and shear. A 'partial penetration' weld goes only part of the way and is sized to the actual load it carries.
For a butt weld in 6 mm plate, a full-penetration weld might run 6 mm deep with a V-prep on each side. For a 6 mm fillet weld on a 6 mm plate, the penetration is around 1 to 2 mm, but the triangular cross-section of the bead provides the strength.
Most gate-trade welds are partial penetration fillets sized to a 4 to 6 mm leg length. Full penetration welds are reserved for structural splices in load-bearing members and for any weld that will see fatigue loading.