This is a quote that says that you should do what you have always wanted to. It says does not plan tomorrow without enjoying today as much as you can. We all are putting a lot of pressure on ourselves as we all want to achieve something. That is important but not letting go the fun of today to worry about tomorrow is not an option as well.
Living in today does not mean that you will not worry about tomorrow. Instead, it means that you should manage what is right in front of you and then worry about the next morning.
It is just about not putting up so much pressure on you. It is how you prioritize your works. No matter what you are doing, maybe you are just a diligent student or a passionate guitar player who want to reach to the top, you should do it every day, and without missing the fun of this day.
This way, you will enjoy your work and life every day, and you will do better, you will perform better the next day. And, the better you will do with your life.
Thus, always keep your priority list in check. Because, you may not do everything in this one life, therefore, make your priority list a priority and leave the fewer priority jobs for the next day. That way, if the tomorrow never comes, if something goes wrong, you will end up in satisfaction as your priority list has gone right.