dependencies - A list of other modules your module depends on. Dependencies should be namespaced in the format {project}:{module}, where {project} is the project name as it appears in the Drupal.org URL (e.g. drupal.org/project/views) and {module} is the module's machine name. Dependencies can also include version restrictions, for examplewebform:webform (>=8.x-5.x). Note that if your module has dependencies on other contributed modules or libraries, these should be declared in the module's composer.json file.
test_dependencies - A list of other modules (in the same format as dependencies) that are needed to run certain automated tests for your module on Drupal's automated test runner ("DrupalCI"), but not needed as module dependencies in general (or that are in development as module dependencies but not finalized yet). Note that you need to have the test_dependencies change committed to your Git repository before you try to run a test that depends on it -- you cannot just put the info.yml changeinto the same patch as the new test. As an alternative, you can also use Composer for test dependences -- see https://www.drupal.org/docs/develop/using-composer/managing-dependencies... for more information.
configure - If your module offers a configuration form, then you can specify the route to this form here. It will then show up as a link in the Extend page (/admin/modules) when the user expands the details.
php: 5.6 - Defines the minimal PHP version that is required for you module. Users will not be able to enable the module if they use an older PHP version. This can be used to avoid errors if your module uses newer functions that did not exists in earlier PHP versions.
hidden: true - This will hide your module from the module list on the Extend page (/admin/modules). You might find it useful to hide a module if it only contains tests, or is intended to serve as an example for developers who need to implement the main module's API. You can make these modules visible by adding $settings['extension_discovery_scan_tests'] = TRUE to your settings.php.