In this passage, Jesus answers the Pharisees who don't get why he doesn't wash his hands before eating. The Pharisees wonder why he doesn't obey the fences that they created around the rules. Jesus wonders why they don't obey the actual rules.
He uses the Corban example. If someone dedicated some property to the temple, then it would pass on to the temple whenever that person died. While the person lived, however, he could still enjoy and use the property however he wanted. These people were using the rule as an excuse to not care for their aging parents. People didn't have retirement funds then, and they depended on their children to care for them in old age. These self-righteous people were using the Corban rule to not care for their parents when they could very much do so because the property was still theirs while they lived.
People cared more about the appearance of righteousness than actual righteousness. Jesus used these examples to show how wicked people are in their own power. "For from within, out of the heart of man, come evil thoughts, sexual immorality, theft, murder, adultery, coveting, wickedness, deceit, sensuality, envy, slander, pride, foolishness. All these evil things come from within, and they defile a person." People are already wicked. They cannot get any more wicked by eating something. God gave Israel those rules to make them see how badly they needed a Savior, not to impose a diet. Now that Jesus had come, he makes people clean and able to come to God without having to die. There is no more need for kosher rules and diets.
This reminds me of what I am teaching. There are three kinds of people. There are people who trust in luck. They believe that events and history happen randomly without any plan or meaning. Then there are people who believe in God. Events and history are specifically ordained by God who knows and loves us and knows exactly why he's doing what he's doing. The third group of people try to be both. They try to use God to get good luck. It doesn't work that way. He is not obligated to something just because you did something. He saves who he will save and has mercy on who he has mercy. He cleans whoever he will, and you cannot manipulate him to serve your purpose. If you do, then you are trying to get saved on your own efforts when you are already so defiled by just one sin that you can never come to God. Thanks be to God that he sent Jesus to be clean for us and that we are clean through him. If only people would just follow Jesus and his will for their lives and not try to live according to what they think is right. Life would be so much better.