Vaginal mucus
Vaginal mucus is a natural substance occurring within the vagina, particularly the cervix, that maintains a certain level of moisture at all times.
The volume, colour, and consistency of vaginal mucus changes according to the period of the menstrual cycle. Both the chemical make-up and the physical consistency (viscosity) change during this time. A slippery, clear, egg white-like consistency usually indicates that the woman is ovulating. Relative dryness usually indicates an infertile point in the menstrual cycle.
Excessive or oddly coloured mucus may indicate various infections of the vagina or pregnancy.
As a sexual response
During sexual stimulation the blood vessels fill inducing vasocongestion. As a result, more fluids (largely sweat) are introduced to the genitals creating a higher level of moisture. This makes penetration of the vagina easier as it serves as natural vaginal lubrication.