On a speed/time graph, the height of the line at any point shows the speed at that moment. If the line is horizontal, then its height isn't changing, meaning that the speed isn't changing. It's constant. The change is zero, until the line starts rising or falling.
Protons and neutrons are all attracted to each other as a result - the strong nuclear force. This is an attractive force that only has an effect over a very short range in the nucleus.
