Right, first post on modern signals:
The top part of the diagram show three-aspect signalling. The signals are spaced apart a bit over the braking distance of the fastest trains on the line so that if a train approaches a signal at the 'Caution' aspect (Yellow, by the way, in railway terms, not amber!) it has sufficient room to stop by the time it gets to the next, possibly Red 'Stop' aspect.
The lower diagram shows 4-aspect signals. They are spaced apart from each other at the braking distance of the slower trains (ie locals) using the line. They then continue to use the single yellow aspect as their 'Caution' warning. The faster trains then use the 'Double Yellow' as their 'Caution' warning and start braking sooner and are thus able to stop.
Both the above are for lines uninterrupted by points and stations, and the function of signals at these places I'll cover next.