"Nail" is the term used to describe the bladed weapons wielded by several enemies, NPCs, and bosses in Hallownest, including The Knight. The Nailsmith in the City of Tears can upgrade the player's Nail up to 4 times if given enough Geo and Pale Ore, which will increase the damage of melee strikes.
Nail Arts are the abilities taught to The Knight by the Nailmasters. Executing a Nail Art involves holding down ATTACK to charge, then releasing while doing an optional second input depending on the Nail Art. Nail Arts deal multiplicative damage, meaning the stronger the player's nail is, the stronger the Nail Art will be.