Modulation is the process of going from one key, like A major, to a different key, E major. The most common modulation is going from a key to it's relative key, like C major to A minor. The notes are the the same so it's very easy to change. The other used is modulating using a relative chord that exists within the key you started with and the key your wanting to go to. Take the A major to E major example. Since the rule is when you write something in a key you end it on the root. Apply that rule with the part your playing in A major, so your chord progression ends on an A major chord. Well there is an A major chord in E major, so you can easily use that chord as a pivot from A major to E major. This technique is commonly called parallel *sumthing*, but it is a process of modulation. With more study in this field you will see that it is possible to move from any key to another.