Can someone explain why we need DAILY_FORECAST and what it is for.

When you are passing data to a new Activity, you need to add it to the Intent you use to start it using what are called Intent Extras. These are key/value pairs, much like entries in a Map, with a String for the key. So DAILY_FORECAST will serve as the key, and the forecast data as the value.

So, why a static constant, and not just a string literal? Since the string is needed to both store and retrieve the data it's best to refer to a single constant and not copy/paste the same string over and over. That way it you need to change it, you only change the static constant, not multiple lines of code with the possibility of missing one. It's best practice, especially when values will be used in more that one place. It keeps constant values in one place near the top of the code where they can be easily changed instead of hard-coded across the code.