From The Heretic Knowledge Vault
A means of securing things, of course.
In this context, the term refers specifically to magical locks, which have appeared in a number of places in the story. First, there are the doors to Meji's Grandfather's house, which Meji complains about in the flashback sequence. (No big surprise there, since Meji complains about everything.) Later, Sarine has to defeat one to get into the Elven Council chambers to vent some spleen regarding Anilis. (We learn from this episode that Sarine still has the security override privileges that she got back in the days when she was in the Viradior. Shoddy operational security, and evidence that the elves get a little, well, complacent over the centuries.) She then applies magic manacles to Sara following the big fight scene in the church so that she and Jon can take Sara along without getting murdered, and reassures Jon about the manacles' effectiveness because of an "extremely complicated lock enchant." (Jon isn't impressed.) Later still, when the doughty crew gets to Tsuirakushiti and has to find a place to lie low, Meji opens the magical door lock at her friend Bani's place (or more accurately, Bani's dad's place since Bani is off in the army) so that they can get in and hide. Finally, magical locks are seen in the cellblock where the elves have stashed Meji and Ellis after bagging them.
We don't know how magical locks work, and they may not all work the same way. When Sarine gets through the one protecting the Elven Council chambers, it looks like she's casting some manner of spell. On the other hand, which Meji gets into Bani's place, she apparently punches numbers or some other code into a pad of buttons. When asked by Jon how a non-magic-user can work one of the things, she says that they "sale (sic) little crystals that you can attune with locks for little kids and tourists" -- hotel-room keys in Tsuiraku?
We also don't know whether there's a Lock "spell" that a mage can cast on the spur of the moment to block a door against pursuit. It stands to reason that there would be, but there's no proof yet; stay tuned. It's not to be confused with a magical Ward, which does exist as a castable spell (Sarine used that too to protect against Sara-cide), need not be placed on a door or other open/shut piece of hardware, and apparently does damage to the would-be evader, which we have not observed magical locks to do.