What is meant by a dropped baseboard?

A dropped baseboard is where modellers construct a small section of their board at a lower level than the rest. The most common reason for doing this is to include a valley in their layout.

Using this method for constructing a valley means that trains can cross the valley on a viaduct at the same height as the rest of the track removing the need to install inclines.

Although it could be argued that inclines are a real feature on many real-life railways, to accurately model one at a prototypical grade would require a phenomenal amount of space. For this reason, a dropped baseboard is a good option for smaller layouts to provide big scenic effects.