Akismet is a comment spam filtering service. Akismet catches blog comment and pingback spam using their algorithms. This algorithm learns from its mistakes and from the actions taken by participating websites. Every WordPress installation comes with two plugins pre-installed. One of them is Akismet. Even though Akismet comes pre-installed, it is not activated by default. You will have to take some extra steps to activate it.
Akismet checks your comments against the Akismet Web service to see if they look like spam or not and lets you review the spam it catches under your blog’s “Comments” admin screen. You’ll need an Akismet.com API key to use it. Keys are free for personal blogs; paid subscriptions are available for businesses and commercial sites.
An API key is similar to a password so remember it and do not share it with anyone. In case you lose or forget your API key, you can always visit Akismet.com and click on the sign in button to access your account page. After signing in you will see your Akismet API key hidden. Click on Reveal link to display your API key.
wpbeginner.com has an article on Akismet.
To learn more about comments in WordPress you can read our blog post here on comments.