Please help with explanation of everything

Can anyone please explain what exactly is happening here? I feel like I totally don't understand what is happening here. I've passed the previous levels with good understanding. However, HashMaps and in general collections seem really confusing to me. I would also appreciate any additional links for readings or any detailed explanation would be greatly appreciated as well.

package com.codegym.task.task08.task0803;

import java.util.HashMap;
import java.util.Map;

/*
HashMap of cats

*/

public class Solution {

    public static void main(String[] args) throws Exception {

        String[] cats = new String[]{"Tiger", "Missy", "Smokey", "Marmalade", "Oscar", "Snowball", "Boss", "Smudge", "Max", "Simba"};

        HashMap<String, Cat> map = addCatsToMap(cats);

        for (Map.Entry<String, Cat> pair : map.entrySet()) {
            System.out.println(pair.getKey() + " - " + pair.getValue());
        }
    }

    public static HashMap<String, Cat> addCatsToMap(String[] cats) {

        HashMap<String, Cat> map2 = new HashMap<>();
        for(int i = 0; i < cats.length; i++){
              map2.put(cats[i], new Cat(cats[i]));
        }
        return map2;

    }


    public static class Cat {
        String name;

        public Cat(String name) {
            this.name = name;
        }

        @Override
        public String toString() {
            return name != null ? name.toUpperCase() : null;
        }
    }
}