Armor and Dexterity, Armor grants you protection but it does have a cost its usefulness to you must be weighed against how much dexterity you have, your bonus number next to your dexterity score is a straight armor bonus, armor caps that with its Maximum Dexterity Bonus. So a high dex character will likely want to forgo wearing heavy armor, or even armor altogether, as it can actually penalize their defense. Whereas a low dex character would want to make use of heavy armor.
Though be warned Jedi/Sith types would want to pump their dex up as they will be restricted from wearing heavy armor most of the time, so Robes or the light Jal-Shey armors are your only options.
Immunities come in two flavors specific immunities (Fear, Disease, Poison, etc.), or immunities to specific attacks or attack types (fire, electricity, etc.) If you have say an 80% immunity to electrical then 80% of the electrical damage you would have done to you is removed, usually after any and all other defenses are penetrated.
Saves, there are generally 3 types of saves Fortitude (based on Constitution), Reflex (based on Dexterity), and Willpower (based on Wisdom).
Fortitude saves are made vs things like Poisons, Diseases, and harmful magical/force effects that would ravage your body.
Reflex saves are made when you set off a trap/mine, or to stave off a magic/force attack that allows for you to be able to dodge out of the way.
Willpower saves are made when you need to resist a mind-controlling effect, like paralysis or mind control.