This is the basic concept of the 555 oscillator.
How it works: the battery charges the capacitor with the resistor to slow the process, when the capacitor reaches the higher limit defined by the 3 resistor network in the package (there are three resistor of 5kohm like the name 555) the first opamp turns on the relay and the capacitor starts to discharge. When the capacitor reaches the lower limit defined from the resistor network the other opamp turns off the relay charging the capacitor in an infinite loop.
Hope this helps bye :-)
-nikisalli
