Chiefs opened at 3 1/2, Line now at 3. What does that mean?

When the point spread shrinks, what does that mean? That more money is coming in on the favorite (Chiefs) or the underdog (Bucs)? I've always had a hard time conceptualizing and understanding this. My guess is when the point spread shrinks, more money is coming in on the favorite and the house is trying to compensate by giving the underdog more of an opportunity to win.