What is redux for if I use a database for my react app?
Database and state management tools such as Redux have different concerns (although they manipulate the same thing: data).
When a client uses your app, it will first retrieve data from the database. At that point, that data needs to be held in memory in order to display it.
You can decide to use the React Component internal state which is component-scoped. Now, this is perfectly fine if you plan on using the data you just retrieved inside the same component.
As your app gets more complex, you'll sometimes need to use the data in different points throughout your app (e.g. if you retrieve the user info, you'll probably need to display it in the header, in the profile page, etc).
This can be tricky to do using the React Component internal state as (if you've tried React a bit) you know passing data is done by passing props down children components.
The common solution when you need to share data between different components is to lift the state up in your app, so you can pass it down to the different components that need it.
This can be tedious and prone to errors as your app grows.
Redux is the solution that addresses that concern. It helps keeping the state you share throughout your app clear and clean by creating a global state that can be accessible anywhere in your application (among other things).
Redux is a state management tool. Redux is for client state, by default it's in-memory only. It is not a 1:1 mapping to your database data but for your views to dispatch actions and then update the store state so other views can react to those data changes. With Redux, the state of your application is kept in a store and each component can access any state that it needs from this store.