nail sticking through; protruding nail
Refers to a nail that has been driven through a board and its point is sticking out the other side. A technical term in woodworking and carpentry.
裏釘が出ているので、金槌で叩いて曲げてください。
The nail is sticking through, so please hammer it down and bend it.
この板
On the back side of this board, you can see several protruding nails.
Compound of 裏 (ura, 'back, reverse side') and 釘 (kugi, 'nail'). The term literally describes a nail that has come through to the back side.