Any two terminal network of voltage sources and resistors can be replace by a single voltage source in combination with a single resistor. This is called the Thevenin’s equivalent circuit:
The first step is to identify the two terminals of the network.
Then to figure out Rth, replace any voltage sources with short circuits and any current sources with open circuits.
Calculate the voltage between the two terminals.
Then figure out Vth between the two terminals.
To figure this out using testing and measuring, it is only necessary to find the voltage at the two terminals with power on, and the resistance with power off.
This can greatly simplify circuit analysis of complex circuits.
Check out videos on YouTube with more detailed information: