What does Mark 6:8 mean?

This is the first time Jesus sends the Twelve to preach and perform miracles, and their provisions appear rather meager. They need to understand that just as God gives them power and authority over sickness and demons, so He provides them with their physical needs. They should expect those they minister to provide for them (Matthew 10:10), just as we are expected to provide for our spiritual leaders (1 Corinthians 9:14).

In Matthew 6:25–34, Jesus tells us to not be anxious about what we will wear or eat. If we seek His kingdom, these things will be added. But "added" doesn't mean "in abundance" or necessarily even to the point of comfort. When the Twelve return, they have been so busy they have not had time to eat (Mark 6:31), and this isn't the first time (Mark 3:20). But they receive enough to get the job done.

Paul addresses this in Philippians 4:13: "I can do all things through him who strengthens me." This doesn't mean he can accomplish literally anything he attempts. It means he knows how to face hardship and abundance (Philippians 4:11–12) with the assurance that God will provide what he needs to do God's will (1 Corinthians 10:13). We are not promised that if we obey God we will always be well-fed and well-clothed. We're only promised that we'll be provided with what's necessary to accomplish the will of God. Even if we die of starvation in our service to God's kingdom, we will hear "well done" (Matthew 25:21, 23).

The inclusion of a staff has led to much study and debate since in Matthew 10:10, Jesus specifically tells them "no…staff." Some think that Matthew's passage refers to a weapon while Mark is talking about just a traveling stick, but the word is the same in Greek. More likely, since Matthew 10:10 is the continuation of Matthew 10:9, the passage means the Twelve may not acquire a staff, but they may bring one if they already own it. The emphasis, once again, is dependence on God, and not so much on the minute details of one's luggage.