Simple MA's 20 and 50 are averaged to produce a mean MA line. I like the smoothness of it, and it can help you stay in a trade in the slow moving trends. Also if you start to see a large cloud in your favor, it could be a great time take the profits.
@PabloFX, I do have this study in .mq5 form. I am not familiar with T-View's policy on cross pollination of platform languages. Before I would share that with you, I would like to make sure I am in compliance.
@lemoon38, You can do this by selecting the Alert Tab at the top of tradingview's chart. Then select the symbol in the top box, and then crosses up, then 20:50mean, then to the right of that "mean" then set your frequency. :) Hope this helps. To code in the alerts it needs to be converted to version 4 pinescript