World of warcraft will write data to a .txt file in your worldofwarcraft\logs directory which contains data on basically every action taken by yourself and everyone in your party or raid as well as all npcs you encounter. You can upload that .txt to a site like worldoflogs.com where it will provide you with usable data that you can analyze. The data on world of logs would be considered a 'parse'. Please respond with your spec so we can direct you to what to look for and how to analyze wol. You can find a multitude of guides on you tube for how to use world of logs but we can direct you to specific things for performance improvement if you tell us what you're trying to do!
This is a good example of how you can use worldoflog parses to improve your gameplay.
The first lava burst mention in that table are your 'full' lava bursts, the 2nd mention in that table are your overloads.
As you can see 10 out of 81 cast lava bursts did not crit. Your average non crit is 45k, your average crit 125k. So you lost basically out on roughly 800k damage there, since as you know all your lava bursts should crit. On top of that you have 2 overloads that did not crit. Your flame shock uptime is 95% (which is decent), so my guess is that flame shock fell off during ascendance or that you are not monitoring your flame shock uptime properly. It happens to all of us now and then that we miss out on one, but 10 is a lot. (900k dmg accounts for a 2.2k dps increase over the course of your 401s fight, and that would 've listed you 2nd on the meters in your raid instead of 4th on that fight)
Point being, by using this parse you could try to improve the way you manage your flame shock uptime by using an addon, have some sort of warning when it drops below 3s left, etc ...
Obviously there is plenty more that you can do with it (dmg taken etc), but you'll gradually start to discover that yourself as you use WoL more and more.