When someone asks you how you are, what do you say? The socially acceptable answer is somewhere between 'fine' and 'good'. Are you really?
Now, it's not a great idea to always burden someone with the full and honest answer to that question, because there's a time and a place for everything, but if we never acknowledge that perhaps we aren't fine and good, then that's not the best, either.
Jesus is being honest with the Pharisees -- they're so focused on looking righteous that they've forgotten to take the next step and actually live a righteous life. Everything may look good from the outside, but they're rotting away. It's like one of those avocados you get at the grocery store that looks great and is completely brown on the inside to the point you just throw it away. It's such a disappointment.
Jesus isn't saying that the exterior acts don't matter. He wants us to insure that our interior life matches. Are we pursuing God in our private lives? Do we have integrity? Are we the kind of people in private that we are in public? What do you do when you have free time? Do our hearts seek God?
All of these are important questions to ask, and to listen for the answers. Discipleship is a lifelong goal. May we never stop seeking God and being humble to listen for the Holy Spirit as it offers correction and guides us towards God.