If a node is locked, it will not cook and its data will not update when inputs or parameters change. The node's data is saved in the locked operator.
See also Flag.
A locked TOP saves the image inside the .toe file as uncompressed image data. The .toe file size will increase by the same amount as the locked image data size. A .toe file saved with locked TOP(s) can be opened by anyone and the image will remain in the TOP(s). For example, a locked Movie In TOP will keep the TOP image stored in the .toe file so it can be opened on any computer, even if the file the TOP references does not exist. If the Movie In TOP is then unlocked, the TOP will try to load the file referenced by its File parameter.
Locking a SOP prevents it from cooking so that manual modeling changes are retained instead of updating parametrically.