To lock is the ability to make something remain the same, or be secure.
The work may make you think of a padlock, or the thing buried in your door that you operate with a key. It may be the set of key strokes on your phone that ensure your phone doesn’t ring Ann when it’s being operated by the loose change in your pocket, or the pen in your handbag.
I find canal locks fascinating. There is quite a palaver in operating them, with a procedure that has to be adhered to fastidiously. A series of locking steps. But with the use of gravity, a boat can be made to either rise or fall for as long as there is water in the canal.
I don’t think the word appears much in the Bible, except with reference to being tied up, like Paul in jail. But there is the use of the word seal, particularly with regard to the work of the holy spirit. To seal would mean to close off to the putrefying action of the air by immersing in oil – or to stamp symbolically with wax indicating not to be touched on the authority of the one who owns the seal.
Whatever picture you like best, the rescue God has made in our lives, no matter how tentative it may seem to you, is sealed, locked, and done so with the utmost authority. Funny how being locked in in this way is the basis for perfect freedom.