It's very simple. The game master tells a story in which all players are the characters involved in the story. Just like in real life, you cannot control the situations around you, you can only control how you react or the actions you take in the situation. Keeping this in mind will greatly help gametime due to the fact that we don't need to stop to tell you that "you can't do that" and have to explain why. So, I will give an example of gameplay:
Gamemaster: "Your sitting in a Cantina on Dantooine. A Sullustan approaches you and attack out of a drunken rage. He pulls a vibroblade and swings down at your head."
Player: "I try to move to the side in order to doge the attack"
Gamemaster: "You will need to roll a Reflex Defense check to see if you can dodge successfully. DC 15 -5 for a drunken hit. The Sullustan is drunk and can't aim as normal. so, DC 10. What is your Reflex Defense score?"
Player: "My Reflex Defense is +1."
Gamemaster: "Ok. You roll 1D20 and add 1 to the result."
Player rolls and the result is 9. The player adds the 1 for his Reflex Defense score for a total of 10.
Gamemaster: "Your roll was either equal to or greater than the DC so you dodged successfully. Now your move."
Player: "I will draw my blaster and fire at his chest."
This is a short example of gameplay. In a real game, it will be much more detailed though. There is a die roll needed to do just about everything you do, even outside of battle. The reason for this is so that no one can be flawless. Someone would like to jump that wall, but they may indeed trip over it in the process, or fall on the first attempt. In play by post, evryone would post that they make it over without any difficulties. The die rolls keeps it more like real life.
Now, to roll the dice, there is a roll dice box just below this window. Click the little drop menu there and choose which die to roll. The n in the box next to that, input the number of those dice you need to roll. After you have typed all you want to say and input the dice info., Hit send and it will automatically post the numbers you rolled. If you want to try it, try it in this thread. I will roll 4D20.