The Sixth House is the house of work, routine, and daily health. It describes the work you do (not necessarily your career — that is the Tenth House — but the daily labor by which you earn your bread), your relationship with your body, and the habits and routines that sustain or undermine your health.
In traditional astrology, the Sixth House is called the House of Sickness (or Service). It describes your physical vulnerabilities, your susceptibility to illness, and the quality of your relationship with those who serve you (and those you serve).
A planet in the Sixth House describes how you approach work and health. Saturn in the Sixth House gives a strong work ethic, but also a tendency toward overwork and stress-related health problems. Jupiter in the Sixth House often gives good health and a generous attitude toward work and service. Mars in the Sixth House gives a direct, energetic approach to work, but also a susceptibility to inflammation, accidents, and stress.
The Sixth House is naturally associated with Virgo and Mercury: the sign of service and the planet of discrimination. When planets are placed in the Sixth House, they take on something of this Virgoan quality, becoming more precise, more analytical, more focused on the details of daily life.