Tags and segments are used to divide a subscriber list into parts that can be targeted by an email campaign.
Tags describe something about the user the tag is attached to.
Tags only need a name, that should describe what the tag says about a user. For example, a tag's name could be "bought-product-A", which would be assigned to subscribers that have bought a certain item in your (web)shop. Users can be assigned multiple tags. Tags by themselves are not used for anything in Mailcoach as long as they are not grouped into a segment.
Tags can be assigned to users manually by clicking their email address in the subscriber list, or automatically when they subscribe through a form, read about this here.
A segment is a group of tags that can be targeted by an email campaign. An example of a segment could be "bought-any-product" or when using negative selectors "have-not-bought-any-product".
When creating a campaign, you can choose to send it to an entire mailing list, or one of its segments: