Graymail is like a bunch of emails that you don't really want, but aren't exactly spam either. It's like when your mom gives you a plate of food and you don't really want to eat it, but you feel bad saying no because it's not gross, just not your favorite.
These graymail emails are usually from companies or newsletters you signed up for at some point, but you don't really care about anymore. They might not be harmful, but they're taking up precious space in your inbox and distracting you from more important things.
Thankfully, there are ways to deal with graymail, like unsubscribing from the emails or creating filters to automatically sort them out of your inbox. Just like you can politely say no to the plate of food your mom offered, you can politely say no to these emails and make your inbox a happier place.