The generator strips accents (café becomes cafe), drops punctuation, turns spaces into a separator, and trims stray separators from the ends. My First Blog Post: 10 Tips! becomes my-first-blog-post-10-tips. Each line is converted separately, so you can paste a whole list of titles.
The slug is the readable last part of a web address, such as my-first-blog-post in example.com/blog/my-first-blog-post. Short, lowercase, hyphenated slugs are easy to read and share.
Hyphens are the usual choice for web addresses because search engines treat them as word separators. Use an underscore only if your system requires it.
See also: case converter · URL encoder and decoder · word counter