If something’s informal, it’s casual and relaxed and doesn’t follow any particular rules or conventions, whether that's a style of writing, or the dress code for your dinner party.
If you have informal plans to see your friend, that means you’ll get together but you’re not sure when — you’ll figure it out later. Informal also means “casual,” like wearing jeans and a t-shirt. Informal writing or speech is the linguistic equivalent of wearing jeans and a t-shirt, with a relaxed tone and simple words. In an informal football club, you don't pay dues to anyone, you just show up at the park and play with whoever is there.