Reiki 3 is known as The Master Level. This simply means you have completed the two previous levels of Reiki and are at the point of learning level 3. It does not mean you have attained enlightenment!! It is simply the name given to Level 3 Reiki.
You will receive the Master Symbol and the initiation that activates it, and you are taught the technique for passing on the initiations for Level 1, 2 and 3 to others.
This does not automatically make you a Reiki teacher, it gives you the technique to pass on initiations.
I believe you need a lot of experience to draw from, both from your daily self practice and from your client practice before you have the grounding and knowledge to go out and teach.