For the first time in Israel's history, while they are wandering in the wilderness, God takes a census. He wants a count of who is among His people, how many they are, how old they are. Of course, God knows all of this already, but it's part of the bigger story He's telling in this scene.
And whenever Israel takes a census, starting with this one, it costs the people something. Every person who is counted is required to bring a monetary offering to the Lord - it's counted among the buyback cost.
To some, this doesn't seem fair. God wants His people counted, then He "punishes" them monetarily for existing? He charges them a fee just because they are part of His people? For those who have doubts about God, this is exactly the kind of evidence they are looking for that God doesn't really care about His people as much as He pretends to; He's just like everyone else - out for His own gain. He wants to see what He can get from His people. He's...duplicitous, at best.
But look at what God does with the money that is collected from this census, from this first census in the middle of nowhere: He uses it to build His Tabernacle.
God takes the money that is collected from the Israelites who are counted and are "just existing," who are "charged just for being alive," and He uses that money to fund the building of the tent where He will dwell among them. He uses it to build His own house in the very place where they are already living.
That's what's so cool about God. He takes our offering and uses it to draw near to us. He uses what we bring to come closer to where we are. When He calls upon us to give to Him, to offer something of ourselves, that is the very thing that makes it possible for Him to dwell among us.
And dwell among us, He does.