A phrase used to indicate two things operating in sync with one another. Synced equipment can be said to be locked up. Though the phrase is often used without discrimination there is actually a distinction between equipment that is truly “locked” and equipment that is merely synchronized. For example, it is impossible for two analog tape machines to be truly locked. Locking implies that clocks are tied together and running as one, as is often the case in digital audio. Synchronization implies that changes in a master device will (eventually) be reflected in a slave device in order to keep it in sync. It’s a subtle but sometimes important difference.
Lock Up also refers to an unfortunate state CPU based equipment gets into from time to time. In this context locking up means “to crash” or “to freeze” where the device in question is no longer responsive and must be rebooted.