@kidgixer
I will try to explain some stuff in here.
Battle points is a system that is used only in online matches. When you perform any action that contributes to the match that is ongoing you earn amounts of battle points. Once you gain enough in that match, you can purchase powerful reinforcements to use on the field to help you win the match. That is it really.
In every match you start with 0 battle points and whatever you earn can only be used in that one match and after it is done it is reset to 0 for the next match so dont let them go to waste etc.
Credits are earned by playing anything include offline mode. The better you do and the longer you play, the more credits you will have. Once you gain enough credits you can permanently unlock powerful Heroes and purchase crates.
Crates contain powerful star cards and crafting resources that you can use to craft even more star cards or upgrade the ones you already have. They can be bought with credits you earn while playing etc. Purchase this when you get enough credits to gain a more powerful arsenal to use in both online and offline content etc.
Star cards are powerful upgrades that you use on your unites to enhance your play style and grow stronger. They can be made by crafting them with crafting parts you get from crates or if you are lucky you can get them instantly from crates. Once you have the star cards you what, collect resources to upgrade them and make them more powerful (Epic purple level being the strongest). Do note as you increase your cards power it will need a higher level requirements to be achieved before you can upgrade them again. First is the resource requirement of course, second is your class level which you increase by having more star cards (every star card represents 1 level and every upgrade an additional level) and your overall ranking which you get by simply playing the game and getting XP.
If there is anything more that I need to explain, let me know!