networking - Twisted pair cable twists and unwanted signals issue

I am confused about one point I have read the following paragraph from the networking book.
“the twists in the twisted pair cable are used to avoid the unwanted signals. For example one twist, one wire is closer to the noise source and the other is farther; in the next twist the reverse is true. Twisting makes its probable that both wires are equally affected by the unwanted signal. This means that the receiver which calculate the difference between the two receives no unwanted signal.”

Now ok I understood the purpose of twists but I am confused about how receiver will calculate the difference when it will receive the signal?. How unwanted signal will be eliminated ?
Another thing that I want to make clear is , I am beginner please provide such an answer that can be understood.

Answer

A 'voltage' as such, is very difficult to measure. In fact, it's hard to even define it. What's always used is a 'voltage difference'. A typical 'AA' battery uses chemical energy to keep a voltage difference of 1.5V between its contact points. A light bulb will light up when a voltage difference forces electric charges to flow through its filament.

Think of a waterfall, the energy of the fall depends only on the difference between the altitude at the top and the bottom of the fall. it doesn't matter if it occurs on top of a mountain or at sea level, as long as the fall itself is the same length.

in old 'single ended' signals (like rs-232, a parallel port, old IDE), bits are represented by the voltage of individual wires.... and a 'reference point' (or ground connection). it's always a voltage difference, but the reference is constant, so it's not always mentioned.

in 'differential signals' (ethernet, 'ultra scsi', any modern serial port (USB, SATA, SAS, FireWire, even PCI-ex!)), each signal is carried by two wires, usually twisted together (or very close traces on a printed board), and the receiver doesn't use a common reference point to measure the voltage difference, it uses the difference between the two signal wires. This way, it doesn't matter if wire A is 22v and wire B is 25V, or A is -10v and B is -7V; it only matters that B is 3V higher than A.