The term that is used in this Scripture for basket is ephah. Ephah is a measurement that is equal to about 5 gallons. Most scholars understand this reference to be in relationship to the measurement and what Zechariah is seeing is the weight of the sin of the people of the land and the need to eliminate that sin. You see that eventually that sin is brought to a certain place and it is destroyed.
But the ephah is not the only vision in this chapter, it is actually the second one for this chapter. The first is that of a flying scroll which is used to judge against those who would steal. Any who steal have as their recompense a curse which is in the form of this scroll which is flying. At the root of these Scriptures and these visions, we are now up to 7 visions in all, is the need for God and humanity to come to terms with each other and for humanity to recognize its sinfulness and its need for a God who will forgive and allow to live.