What does Mark 7:13 mean?

The scribes from Jerusalem and the local Pharisees have accused Jesus' disciples of breaking the Jewish traditions by not washing their hands before they eat. Instead of defending His followers, Jesus tells the accusers that their devotion to such petty, manmade traditions exposes the fact that their honor of God is all talk and no heart.

The scribes of old created the oral law in an attempt to protect the Jews from breaking the Mosaic Law. Such efforts not only don't work, but the results of applying the oral law often directly contradicted the Mosaic Law. Jesus uses the example of inflating the importance of a rash oath (Numbers 30:2) until a man is forced to disobey one of the Ten Commandments (Exodus 20:12) and a law punishable by death (Exodus 21:17).

The "word of God" here simply means God's utterances. In this case, it refers to the fifth commandment, not Jesus or Scripture as a whole.

"Making void" is from the Greek root word akuroo. It means to remove all power and authority. What started as disassociating from the law (Mark 7:8) and grew to rejecting its authority (Mark 7:9) has come to fruition in removing its authority. Although we cannot really nullify the authority of God's words, we can make them functionally useless by living and teaching as if our standards are greater than His. By doing so, we take His name in vain.

Jesus mentions that He could have given other examples. A possible reason that He gave this one is that refusing to support one's parents is a crime worthy of capital punishment.