what does it mean when there is an icloud icon in itunes that has a down arrow in the middle of it?

I am having trouble with my itunes account because some of my song will not let me play them because they have an icloud icon with a down arrow in the middle and whenever I click on the song to play it a message pops up and says "You must authorize this computer before you can download the previous purchase". what does that mean and how do i fix it?