Simulation of half-life (exponential decay)

This applet illustrates the progression of decay we observe if we begin with 10 units of our material which has a constant rate of half-life decay. Click "Start" to begin the simulation. What do you notice? What is happening in this simulation?

Half-Life

What does half-life represent? What do the following represent? first half-life second half-life third half-life and so on...