Music I purchased on I Tunes is not available when I open ITunes in I Movie. I Movie says those files are "protected" and not accessible. What does that mean and can I change them to "unprotected" so I can use them in IMovie?

Most of the music that I purchased on I Tunes I bought and downloaded on my old I Mac which I replaced with this I Mac about 9 months ago. When I"m using I Movie on this I Mac and try to open I Tunes tab in I Movie, most of the files are not accessible because I Movie says those fields are protected. What does that mean and can I change those files to unprotected so they show up in my music in IMovie?

This I Mac is OS X version 10.9.5 Maverick, 2.7GHz Intel Core 15 processor, 8GB 1600MHz DDR3 memory.