Norton's Theorem states that It's possible to simplify any linear circuit,no matter how complex to an equivalent circuit with just a single current source and parallel resistor connected to a load. A normal circuit looks like this:

Any 2 terminals of a network can be replaced by a current source and a resistance in parallel to it.

Norton's theorem:

Norton’s Theorem states that it is possible to simplify any linear circuit, no matter how complex, to an equivalent circuit with just a single current source and parallel resistance connected to a load. Just as with Thevenin’s Theorem, the qualification of “linear” is identical to that found in …