Nuclear force
This binds the nucleus together. It acts between the constituent nucleons, and is a strong attractive force that can overcome the electrostatic repulsion. However, the range that this force can act over is very small (to the order of a few neutrons or protons).
Electrostatic repulsion
This acts between protons - like charges repel, and so the protons naturally want to move away from one another. This is overcome by the nuclear force but has a much longer range.
This means that when a nucleus gets big enough (larger than 12 or 13 nucleons in diameter), the electrostatic repulsion becomes stronger than the nuclear attractive force and the nucleus becomes unstable. For this reason, the energy needed to break a larger nucleus apart is less than that needed for a smaller one. Larger nuclei are less tightly bound together as the nuclear force is just too weak to hold on. However, a similar effect occurs when elements become very large - simply needing to break apart very large nuclei takes up more energy than breaking apart an intermediate-mass nucleus such as iron or nickel.