The 555 can be wired as an OSCILLATOR. It needs 2 external components - a resistor R and a capacitor C. These are called TIMING COMPONENTS. The diagram below shows these two components:
The capacitor charges via R and when it reaches 2/3 of rail voltage, pin 7 shorts the capacitor to ground. This means the capacitor charges slowly but discharges very quickly. An improved layout is shown below:
The capacitor charges via R (plus the top resistor) and discharges via R (only). If the top resistor is small compared with R, we can neglect it, so that C charges via R and discharges via R at about the same rate.
The top resistor simply separates pin 7 from the positive rail as pin 7 shorts to ground to discharge the capacitor during part of the cycle.