access_token
When you want to use Mastodon API, you need access_token for your application. Using this site, you can get new access_token of your Mastodon site.
You can see all applications you use in Authorized Application of Settings.
This page uses your client_id, client_secret and access_token, but the site is pure JavaScript SPA site; no server-side applications, no external scripts, no tracking your activities.
If you can't trust this site, you can copy this page and execute it anywhere. This page uses Vanilla JS, so you can easily confirm what it does.