What changes
A slug is the readable end of an address, such as /creme-brulee-a-recipe. Letters lose their accents (é becomes e, ß becomes ss, ø becomes o), everything is lower case, and runs of spaces and punctuation become one hyphen, with none at either end. Apostrophes are dropped rather than split, so don't becomes dont, and & becomes and.
Shorter slugs
Longest trims the slug to a number of characters, cutting at the end of a whole word rather than through one. Drop small words removes words like a, the, of and to, unless the title is made of nothing else. Underscores joins the words with _ for systems that want it.
Many at once
Put several titles on separate lines to get a slug for each, on the same lines, ready to paste into a spreadsheet.
Questions
Hyphens or underscores?
Hyphens, for web pages. Google reads a hyphen as a space between words and has advised against underscores in URLs, which it can read as joining the words into one.
What happens to Greek, Cyrillic or Chinese letters?
They are left out, since a slug here keeps to a to z and 0 to 9. A title written only in those letters gives no slug, and the line under the box says which line it was.
Should I change the slug of a page that is already published?
Only with a 301 redirect from the old address to the new one, or links to the old address and its place in search results are lost.