Okay kiddo, have you ever seen a big, green field? Well, all those plants in the field are called grass. And you know what? Grass is actually a type of plant that is used to feed animals and make different things like paper or even houses.
Now, when people say "all flesh is grass," it means that everything that is living on this earth will eventually die and turn into grass or other organic materials. Just like how the grass in the fields grows, animals and people will eventually pass away and their bodies will become part of the earth, which helps new plants and organisms grow.
So even though we might be different from grass, we have something in common: we all eventually become part of the same cycle of life and death that is happening constantly on our planet.