At the top left is the player window. It displays your character's health, power, and concentration. These three colored bars will change in real-time whenever you become hurt or heal, and whenever you consume or replenish your mental resources. You'll refer to this information frequently to judge how healthy or distressed your character is, because it's never a good idea to let your character die. Beside your character's info will be the same information about your enemy, if you have selected one. These are discussed in more detail in the Beginner Guides on Understanding Your Character and Understanding Your Enemy .
Particles - This section allows you to adjust the number of particles displayed during spell effects. In some cases a lower setting may improve performance. You can make adjustments for Spells, the Environment (Env), and Player particles. You should set these for Me + NPC, and just pick a low density if you are using a slow computer. A lot of AE spells have particle effects that flood the area they affect, and sometimes seeing these particles is the difference between life and death. I wouldn't use anything other than Me + NPC unless you like to see a LOT of flashy particle effects.