Twitter card tags set the title, description and image X shows when someone posts your link. You get them here in one block with your title tag, meta description, canonical link and Open Graph tags. Type a title and description, check their character counts, fill in the other three fields, and paste the block into your page's head. There's no signup, and the tags build in your browser as you type.
How to use it
Type the page's title and description first. Those two have character counts, so you can see the length before you copy anything. Then fill in the other fields and the Twitter card and Open Graph tags fill in from what you typed.
Copy the whole block and paste it between the head tags of the page. Do it once per page, since each page needs its own title and description.
What the generator doesn't do
It doesn't fetch your live page, check the tags already on it, or show a preview of the card. It writes the tags from the five fields you fill in, and you paste them into the page yourself.
This meta tag generator is free with no signup. SearchBlueprint, who make it, sell SEO planning for websites: which searches each page should answer.