Lock has a wide variety of meanings - a lock of hair, enlocked arms, a means to secure something. But I couldn't pass by the meaning that has local significance:
Lock: an enclosure (as in a canal) with gates at each end used in raising or lowering boats as they pass from level to level.
The Hiram M. Chittenden Locks known locally as the Ballard Locks are are part of the Lake Washington Ship Canal, connecting the fresh water Lake Washington to the salty Puget Sound. These were built between 1911 and 1917. Today they continue under the control of the Army Corp of Engineers transporting both pleasure boats and commercial boats. For more on Locks - here is the Wikipedia entry.
It is always fun to watch the boats enter the lock. It takes a while until boats are secured along the sides and the door close. Then water will fill or empty from the lock to raise or lower the boats depending upon whether they are going to or from the Sound. You can walk along narrow walkways above the massive doors which gives you an excellent view down on the boats. The Locks are a favorite of both locals and tourists.
There is also an excellent garden on the Locks property. The area is very temperate since it is close to the water and it is a good place to see plants exotic to Seattle such as palm trees and Princess Tree. There is also a large terraced grass hillside overlooking the locks which is a favorite place to picnic or just watch the boat traffic.
Here are a few more photos of the Locks.