Let us see if I can explain this…
Stacks can be anything that has more than 1. If you have 4 Singularities, then you have a stack of 4.
You have to watch stacks of Debuffs too.
Certain attacks grant different stacks other attacks use those stacks or get added damage or armor bonuses when used with stacks. So you want to give priority to the attacks that grant stacks and then once you have the stacks you want to give priority to attacks that give increased damage when using those stacks.
Focus is a jedi knight’s resource. Unlike other class they start with zero resource and build it up. See the yellow bar below the health bar. At the start of the fight you want to give priority to building this up as fast as possible. Certain attacks require focus. See those little yellow rectangles on the Focus Bar.
So in the screenshot you can see the stacks I have above Mar'ina's name where the normal class buffs are. You can also see the debuff on the left side of her health bar (red). Below the health bar is the yellow focus bar. She has 8 points in focus. She also has two stacks one with 26 in it and another with 4. I know certain abilities need 30 of that type of stack to use. If I want to use that ability, then I know I need to give priority in my rotation to the ability that grants that stack in order to get to the 30 needed to use the ability I want to use.
Know how to get and maintain your resource and stacks and then know what abilities give the biggest bang for the buck is key to becoming a decent player. Otherwise you running out of focus and stack and you are forced to use your lower dps default attack to build up Focus.
I wouldn't worry about this overly right now. Simplify it, find a rotation that you are comfortable with and semi-follow those links I gave. You hit that rotation 50% of the time and you will not have any trouble getting to 50. After 50 in a group setting is when you have to really learn your rotation in order to not be carried through content. So in other words...find out what works for you solo level and stick with it...just know you may have to adjust your style at 50 or in a group setting.