Please help my cluelessness- ISBN numbers

Hello all,

Excuse my cluelessness but I'm self publishing my book in August through Blurb first and then going from there, but my formatter/editor and general handler of things I don't understand was talking to me about ISBN numbers... Now I understand that I need to pay around £126 for one!? I'm a student and recently lost my job and haven't found another one, so I can't exactly afford it right now. We have a plan but can somebody out there please explain the ISBN thing to me because I still have no idea why I need it so desperately or how I'm supposed to acquire it. I just know I'm mad at it! xD thank you!