The simplest 555 oscillator takes output pin 3 to capacitor C1 via resistor R1. When the circuit is turned on, C1 is uncharged and output pin 3 is HIGH. C1 charges via R1 and when Pin 6 detects 2/3 rail voltage, output pin 3 goes LOW. R1 now discharges capacitor C1 and when pin 2 detects 1/3 rail voltage, output pin 3 goes HIGH to repeat the cycle.
The amount of time when the output is HIGH is called the MARK and the time when the output is LOW is called the SPACE. In the diagram the mark is the same length as the space and this is called 1:1 or 50%:50%. If a resistor and capacitor (or electrolytic) is placed on the output, the result is very similar to a sinewave.
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