Faith is the currency of God's kingdom. Without it we cannot do business in God's economy.
The disciples have already been sent out and been used by God to heal the sick, cast out demons and cleanse the lepers, so why is it that they cant seem to cast out the demon from this boy? I think part of the reason is because they had already been used by God to do all those things and what it did was made them confident. However the confidence was not in God and His power but the disciples had come to believe that the power was in them. The faith they were lacking was the faith in the power of God to cast out the demon. They may even have said the right words and done all the right actions but they were operating in a self-confidence rather than a God-dependence.
Jesus tells them that they need the faith of a mustard seed. It does not require a lot of faith to do the will of God, but it requires faith in God. Mustard seed faith is persistent faith. It continues to grow and become productive because it never gives up. It stands firm in the face of great obstacles. It is the quiet assurance that God will accomplish His will in and through us if we stand firm and believe in Him. That is really the who understanding of faith - it is not some complicated or mysterious thing - it is simply believing God and His Word no matter what the situation looks like. It is trusting in the power of God to bring about His plans and purposes and not depending on our own doing. It is partnering with God by allowing Him to lead and us following with trust that the end is something good.
Have faith in God and in so doing live a life that is pleasing to Him.