scraping multiple pages of a website.

[Blue Dog]

Hi Buran, I took you code apart peace by peace. I know every bit of it now, but for this.
Tell what this does how it works

url = '{}{}'.format(base_url, letter)

what does '{} {}' this do
I try to find .format in the requests Mod. but could not.
what is and what does .format do
I know what base_url is and I think letter is the letter at the end of the web page address. If you can, I need to know everything about this line of code.
Thank you
Renny