This rewrites the scheme at the front of every URL in a list from http:// to https://, leaving the rest of each address untouched.
Why lists still contain HTTP
Old backlink exports, citation records, directory listings and archived documents were written when HTTP was normal. The sites have since moved to HTTPS, but the recorded URLs did not update themselves.
What the conversion is for
Comparing an old list against a current one requires both to use the same scheme, or every row appears to differ. Converting first removes that false difference.
One caution
Converting the string does not make the target work. It assumes the destination genuinely serves HTTPS, which is worth confirming for older or abandoned domains.
Try it: Convert HTTP to HTTPS on SeoWolf's Notepad.