In many cases we don't care about the details within a circuit, only how it appears from the outside looking in. Here, a group of resistors has the exact same behaviour as a single resistor. You can "lump" the resistors by following the path of current: Any thing in series adds up. Any time it brances is in parallel.
All in kOhms:
R_eq = 1 + 4||(3 + 2||2)
= 1 + 4||(3 + 1)
= 1 + 4||4
= 1 + 2
= 3 kOhms
When you simulate, you can see that there's 1 mA going into the resistor network and 1 mA going into the equivalent resistor.
