Akismet is a leading spam filtering service that's used by millions of websites. For non-commercial websites, you can obtain a free API key (you can also pay as much as you want). Akismet integration is completely optional in Commento; if you do not want to use their service, comment will not be filtered. However, if you do choose to enable Akismet integration, be advised that all comments will be sent to Akismet's servers for spam detection.
To do this, go to their website and signup for an account. Choose your appropriate plan, and then go to your dashboard. Here, you'll find your 12-digit hexadecimal API key. To make Commento use this key, either set an environment variable though export AKISMET_KEY=abcdef012345, or use a .env file (see README for more details).