An astrological natal chart is considered to be a symbolic blueprint for a person's life, defining the main issues that will concern them, the qualities those issues will take on, and where they'll play out in their lives. However it is not their fate set into a tablet of stone. How the person deals with those issues as they arise is always a matter of choice, and within the prominent general themes there are any number of potential outcomes. Opportunities can be taken or lost; challenges can be met or avoided.
A chart for an event, or a thing like an essence, is not dissimilar. It describes the nature of the event/thing and its qualitative relationship to the various energies prominent in the collective psyche at the time.
The chart itself is a schematic representation of the positions of the various planets of the solar system at the time of the event, mapped onto a circle.
The circle (= zodiac) is divided into 12 equal 30° segments, or "signs", representing each monthly stage of the sun's apparent journey through the sky during the course of the Earth's annual orbit of the sun, beginning with the spring equinox. At this point, astrology splits into two camps as to where the spring equinox is mapped onto the circle. Because of the precession of the equinoxes, the Sun's position at the spring equinox is a constantly moving point in relation to the constellations. In Tropical astrology, the spring equinox remains fixed at 0° Aries, tying the zodiac to the Earth's seasons and equinoxes. In Sidereal astrology, the spring equinox is mapped onto the actual position of the Sun in relation to the constellations at that time, tying the zodiac to the position of the constellations. Around 2,000 years ago when the spring equinox was at 0° Aries, there was no difference between the two systems. Today there is about 24° difference between them. Strange though it may seem, both systems appear to work with equal validity. Most western astrologers use the tropical system.
The sign in which the Sun is found is the one used in populist 'astrology' and is the sign people mean when they ask what sign you 'are'. However, although it's one of the stronger indicators of the chart subject's character, it's little short of ludicrous to consider it without other equally strong personal indicators like the Ascendent and the Moon. It's no wonder that such 'astrology' is widely ridiculed.
As well as the signs, the circle is divided into another 12 segments, or "houses", beginning with the exact position of the eastern horizon on the circle at the time and place of the event (= Ascendent, or "rising sign"). Because it's a feature of the Earth's rotation on its axis, the position of the Ascendent moves through all the signs of the zodiac once every 24 hours. There are several systems for dividing the rest of the circle up into its 12 houses, and the 12 segments are not necessarily equal, but they all represent various areas of life and types of relationship going counter-clockwise from the innermost and personal to the outermost and universal.
Broadly speaking, within the totality (individual, event, etc) which is the subject of the chart (the which of the equation), each planet symbolises a psychodynamic theme or archetype (the what) which centres on the mythical role of the Greek/Roman deity that is the planet's namesake. The planets closest to the sun (and hence fastest moving in terms of relative position) represent issues that are more personal and individual to the subject of the chart, whereas the more distant and slower moving ones tend to define aspects that have more collective and generational colouring. The sign the planet is found in symbolises the main qualities of how that archetype will play out, and the house that it's found in is the area of life where its influence is strongest. In addition, the spatial relationships between planets (= aspects) that conform to equal divisions of the circle (eg. 180°–half, 120°–third, 90°–quarter, 72°–fifth, 60°–sixth, etc) have relevance. These patterns between planets show with what other influences each archetype is associated. The number of degrees apart, and whether or not the planet falls at the midpoint between another pair, defines the various qualities of the association and the way in which it might tend to play out.
Transits and progressions are methods of relating present time back to the birth chart and signal when each archetype within the chart is likely to be triggered and hence most prominent.
There are many more additional layers of subtlety, and many different schools which pursue different methods of analysing a chart, giving it the potential to be used as a symbolic map of phenomenal depth and complexity.