The amplitude of a trig function defines how much the graph is going to be getting stretched or compressed on the y-axis. Take for example the following function.

$$sin(x)$$

On this function, no compression or stretching on the y-axis is happening but if you add an amplitude of 3 the amplitude is going to stretch the function values up to the 3 mark on the y-axis.

$$3sin(x)$$

The same concept applies to compressing the function for a value that is smaller than one.

$$\frac { 1 }{ 2 }sin(x)$$

If you now overlay all three functions together you will see that the function still have the same intercepts on the x-axis just the values on the y-axis have changed everything else has remained the same.