What does the "Caution" box mean on the sectional near Half Moon Bay, California?
This one is a bit confusing and interestingly there is nothing in the FAA's sectional guide on "Caution" boxes although they do appear on other charts as well. 

This one in particular is a warning that you will need monitor the radio for the transition routes below 2000 ft. If you can zoom in or get TAC level map resolution you will see a different warning 



(source)

If you note a bit south of half moon bay the chart makes note of the fact you should be using the TAC in that area: 



(source)