Sufficient unto the day is the evil thereof

Imagine you have a big project due tomorrow and you know it's going to be really hard and stressful to finish on time. But instead of worrying about it all day today, you should focus on doing the things you need to do today and not think too much about tomorrow. This is because if you worry too much about something that might go wrong tomorrow, it can make you feel scared or unhappy today for no reason. So, just focus on what you can control today and deal with tomorrow's problems when they come. That's what "sufficient unto the day is the evil thereof" means - just deal with today's problems and don't worry too much about tomorrow's problems.