An ephemeris is a day-by-day listing of the zodiac positions of each planet. Ultimately, many important astrological techniques boil down to comparing your birthchart to the ephemeris and knowing how to pull information out of these tables. To most people, this is just a boring table of numbers. To me, an ephemeris is a fascinating story just waiting to be discovered.
Please check out the Astrology Help Pages for additional background information. In particular, there's a help page for explaining the notation for zodiac positions used in the ephemeris tables, plus some thumbnail sketches of the planets themselves.
The planetary positions listed are for 0:00 CST (midnight), my local time zone. That is the same as 6:00 GMT. Time zones are explained on the Notation page.