The Archive Title module is in the Archive modules category in Themer, which appears on when you're editing an Archive-type themer layout.
The Archive Title module displays the title of the archive. If it's an index (general archive), it displays the title Posts. If it's a category Archive, it displays the name of the category as the title. And so on.
The Archive Title module is a "shortcut" module. It's actually a Heading module with a field connection set to Archive title in the Heading field. When you open this module for editing, you'll have the regular Heading module settings, such as:
- Adding an optional link
- Style settings for text, including font family
- Special settings for mobile devices
- The usual Advanced tab settings