An astrological chart is a map of the positions of the planets as seen from earth1. The lines you see radiating from the center of the chart are the doorways, or cusps, of the astrological houses. They change as the Earth rotates on its axis, therefore the houses depend on the birth time, and their positions in the chart will not be accurate if the birth time is off.
The houses represent the mundane position– in other words, the relationship of the planets to the Earth at the time of birth. As the Earth rotates, the Sun appears to rise and set, and the houses also change. Therefore, you need a pretty accurate birth time (within 10 minutes or so) to be certain of the position of your house cusps.
The thick horizontal line which runs through the middle of the chart represents the horizon, or sunrise and sunset. This line forms the cusp of the 1st house (you) on the left side of the chart, and the 7th house (others) on the right side. It divides day from night.
If you were born at night, the Sun will appear below the line in your chart. If you were born during the day, the Sun will appear above the horizon line.
The thick vertical line which runs through the middle of the chart represents the meridian, or noon and midnight. This line forms the cusps of the 10th and 4th houses, representing the “outer you” (10th house), and the “inner you” (4th house).
Run your finger around the chart, counter-clockwise. The first line your finger crosses before going into a house is called the cusp of that house. If you follow that line to the outer circle of the chart, you will probably see a glyph for a sign and a number between 0 and 29. The glyph tells you the sign on the cusp. That sign tells you the nature of the affairs of that house in your life. (The number tells you how many degrees into the sign you are. 0 is the beginning, and 29 is the end.)
Empty house? Don’t worry, it doesn’t mean the house is screwed up. In fact, it means that house is normal! Planets in a house tell you where the issues are in life.
1 Unless notated otherwise, an astrological chart is assumed to be a geocentric (earth in the middle) chart. There are techniques that incorporate heliocentric (Sun in the middle) charts as well. But those are far less common.