The bokeh.core package provides modules that are useful for implementing Bokeh itself. Documentation for all of them can be accessed through the sidebar menu. Most of the modules here are probably not of general interest to most users. However, some are more useful, especially to anyone writing custom extensions to Bokeh. These are listed below:

Properties on Bokeh models support automatic type validation, including specifying and validating enumerated values. There are many enumerations used across Bokeh. This section has documentation on all the ones that are built-in, as well as information about how to create new ones.
The fundamental building block of Bokeh apps and documents is the Bokeh models, for instance plots, ranges, axes, etc. Bokeh models are comrpised of properties, which are named attributes with specified types. Model properties can automatically validate and serialize themselves. This section describes all the property types that can be attached to Bokeh models, which is of interest when creating custom extensions.
Some collections of properties appear together often. Property mixins are groups of properties, such as a fill_color and fill_alpha, that make up a single unit that can be easily applied to Bokeh models all at once.
When serializing a Document for use by BokehJS, the Bokeh python library attempts to detect potential or actual usage problems. These are reported as validation warnings or errors, which have unique numeric codes and names associated with them. This section is useful to find out more specifics about such warnings and errors.