Imagine you and I like to talk and we write each other post cards. And we send them via a local courier business run by a 16 year old who wants summer money.
4G is where we use their service so send a card each day and its consistent and works across our neighbourhood.
5G (non millimetre wave) is about the same as 4G but the courier us using an electric bike rather than their normal bike.
5G Millimetre is when this teen hires 4 other kids and they all are using go-carts to go back and forth from our houses to deliver each post card.
The up side is its fast as hell, the downside is go-carts can drive on normal roads.
Using a bike might be slower but they can use public roads and we can connect outside of our neighbourhood. Its slower but it generally works at longer ranges.