CSF is the abbreviations of Cerebrospinal fluid, a protective fluid that surrounds and supports the brain and spinal cord.

A clear watery fluid whose normal contents include gluco se, salts, enzymes, and some white blood cells (but no red blood cells).

This fluid moves within its cavity, typically beating at 6-12 beats per minute, though this can rise to 12-50 beats per minute , such as in the case of a person who has a "pulsating" headache.

The functions of CSF are as follows:

1. Bathes the brain and spinal cord
2. Allows nutrients and waste products to diffuse between the blood and the brain/spinal cord.
3. Protects the nerves against mechanical damage

Cerebrospinal fluid is also the subject of cranio-sacral therapy, which is a huge subject in its own right.
A list of books about the topic CSF