Can someone explain what a missing files host is (on Google/Blogger)?

I just moved my blog (Blogspot) to a domain name. I now have to choose where I want a “missing files host” or not…before I can complete the transfer to the new domain name. (I am not tech savvy.) It says:

If you specify a missing files host, Blogger will look there if it cannot find a specified file on your regular domain (have no clue what this means?)

I am supposed to enter it like this (where the x’s are):

http://xxxxxxxxx .Idon’tunderstandthistechstuff.com

Your missing files host must be a subdomain in the same domain as your blog.

Okay, I don’t understand what I am supposed to put in the “x” spot or what a missing files host is/does, nor what a subdomain is.

Should I sign up for this or not? I had a blogspot address before as in : www.idon'tunderstandthistechstuff.blogspot.com and now am shifting over to a plain domain name: www.idon'tunderstandthistechstuff.com

Can someone—-please please please——explain this to me and what I should do? In clear steps? If I click “no” to missing files host what would happen?

I would appreciate any information and/or help. Thanks so much!