Q. What does "Claim" mean? Do I have to claim every paper?

"Claim" is to indicate the paper is related to you (i.e. you are one of the authors). Ideally, IRIS should hide the Claim button if it detects the paper is yours or that you have already claimed it, but it is not able to differentiate. So, check your list of publications, you do not need to claim a paper if it already appears in the list of your publications.