Selectors in Redux, are functions which pluck out, and return the part of the Redux state which you are interested in!

As a simple example; if you have a normalised redux state tree like this;

{
    posts: {
        12: { id: 12, content: ..., },
        ...
    },
    ...
}

you might write a selector function like the following, to retrieve a post by its id:

const getPost = (state, id) => state.posts[id];

Selectors are especially useful when you have to compute derived data, based on the given conditions, the first example in the first page which you've shared is a good example.

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Once you realise that you can avoid a lot of unnecessary derived data computations, you can use a library like reselect which lets you write memoized selectors to tackle the above problem.

You can read the following answer to learn a bit more about reselect: