Really basic question about WordPress hooks

Hi, I read somewhere that you can provide general wordpress support (correct me if I’m wrong!).

I am writing a custom plugin. My question is can you describe a super simple function for the functions.php file that takes the_content() and add a bit of text on the end. Let’s just say for example I wanted to add ‘<p>peace</p>’ to the end of every single post on my site, how would I create that function?