Just what the title says.
The lock would inhibit the slider from moving if the user clicks on it. Even if the user adjusts the other sliders up or down, the resource(s) with the lock enabled will not move until it is disabled again.
The lock feature would work the same to the percentage sliders on the Dome UI for the construction, research, and military sliders.
The benefit of this may sound minor and perhaps it is since we're talking about shaving a few seconds off colony management when fine tuning the sliders, but in the long run it's the minor features that really add up to a great UI experience.
-Food for thought