The Text Editor module provides a framework for managing editors. To use it, you also need to enable a text editor. This can either be the core <a href=":ckeditor">CKEditor module</a>, which can be enabled on the <a href=":extend">Extend page</a>, or a contributed module for any other text editor. When installing a contributed text editor module, be sure to check the installation instructions, because you will most likely need to download and install an external library as well as the Drupal module.