Thrive Optimize is an add-on plugin that sits on top of Thrive Architect and gives it split testing capabilities. Once installed, you can create split tests which allow you to create 2 (or more) variations of a particular page. The plugin will then randomly route your website traffic to each landing page variation, letting them compete against each other and telling you which one works best.
To determine which worked best depends on your chosen conversion goal. This could be someone signing up for your list, visiting a page on your site, or purchasing a product.
Thrive Optimize is not a standalone plugin that can work on its own. Instead, it works in conjunction with Thrive Architect, which is a page builder that lets you design rich looking landing pages for WordPress.