Hey Charlie!

Here's how the Zendesk Glossary defines "comments":

When a ticket request is submitted it contains a subject and a description. All follow-up communication on the ticket is contained in comments. Agents and requesters can add comments to the ticket. Comments can also be added by automations, macros, and triggers. There are two types of comment: public and private (or internal). Everyone, including anyone copied on a ticket, can see public comments but only agents can see private comments.

In short, comments are the followup responses made on tickets by agents/admins and end-users, both public and private. Agent comments specifically counts those followup responses made by agents/admins, ignoring all comments made by end-users.