Project: its purpose is to organize your tests on a macro level like a team, a project or to a granular level for parts of your website or app.
Tests: inside the project you will have your tests: like for example a test for your start page.
Test variations: for the start page test itself you'll want to have tests running on different browsers/devices. Than a test will consist of multiple test variations.
Test group: (feature coming soon). You can group your tests by a certain criteria inside a project.
Filters: filter your tests by failed, unreviewed, group, browser, user that triggered the test, accepted the test.