If the layer is moved at the bottom, then the value would be the number of this last layer. If the number of the layer is 3, then the index expressions added on a property of that layer returns avalue of 3.
The index expression just gives you the number of the layer. The random expression generates random values, like for example random(0,720), which randomizes the property of the layer (position, scale, rotation etc) between 0 and 720 every single frame. There are a lot of other useful expressions that we could use to make work a lot easier, such as the random, the index, and the time expressions.
