An alliance for healthy cities is like a big group of people who live in different cities and work together to make sure everything's healthy and safe. They do things like making sure there's clean air to breathe, making sure there's clean water to drink, and making sure there are safe places to play outside. This big group of people talks to each other and shares their ideas and their knowledge to make sure they're doing everything they can to keep everyone healthy. Think of it like when you're playing with your friends and you all work together to build the best sandcastle you can - the alliance is like that, but for keeping people healthy in cities.