What are BCAA supplements for? Simple explanation please. How can they help me?

I've always struggled with having a high metabolism and its always hard for me to gain muscle mass because of it. I also have a small frame and it sucks. There are times where i feel like im making progress. I feel like I gain some size. Then after a little while like a week I feel skinnier. What makes things worse is that I have a small appetite and I get full quickly. Its hard to have to forcefeed myself everyday. Will BCAA give me any benefits?
What are BCAA supplements for? Simple explanation please. How can they help me?
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