Rainy Day is talking about that day that a person will not be able to work hard or will not have the strength to do many works to feed for theirselves and thier families so they should rather do the hard work in their youth days so that's when that day comes,they will not go Hungry or suffer in terms of financials. The song is also talking about how to live a good and healthy life and also giving hope to the youth ....typical example is the lockdown, where movement is very slow so those who worked hard and saved alot of money will not complain of hunger .... 💪