All 10/100/1000 Base-T copper wiring are composed of two used pairs – transmit and receive.
A hub or switch usually has these reversed from typical end objects (PC’s, Servers, Routers, etc.) so that the send pair from one goes to the receive pair from the other and vice versa.
While many newer devices have auto-detect (and auto-switch) for this, most PC’s and such do not.
If you’re connecting two pc’s to each other without an intervening switch or hub, you need a cross over cable.
Internally (to the cable) all it does is connect the pair of wires 1 & 2 to the pair of wires 3 & 6, and vice versa the other direction.