Lockbox key is a key and quest item in The Witcher 3: Wild Hunt.
It's needed to open a lockbox in the barn at Reardon Manor. The strongbox was owned by Dolores Reardon who had left it behind when she married and moved with her husband. After her husband passed, she returned to the area in 1272 but found the manor was now overrun with monsters.
If Geralt comes along and helps her out, she'll give him this key and states that everything in the box is his payment for helping her. Before he can get his reward though, Geralt must first find the key to the barn.