A lock block is the part of the door that the lock will be built into. In our standard core doors with a cellular structure, the lock block will be an additional solid piece of wood that you may find on one or both sides of the door, depending on the specific door design.
To identify where the local block is / lock blocks are, just take a look at the top or bottom of the door and there will be either a key/keyhole illustration or the word ‘Lock’ printed on the relevant side.