I have a 94 buick lesabre and it started making a clicking noise.?

A mechanic said it was my harmonic belt. What is that and is it possible for me to replace it myself. Im not experinced so would it be out of my league to replace it myself?? When i turn the belt that crosses over the alternator it stops by making a clicking noise. When i turn on the car that when the clicking noise gets faster and louder. Any suggestions??