When you configure a table in $TCA and add a field of the type "text" which is edited by a <textarea>, you can choose to use a Rich Text Editor (RTE) instead of the simple form field. An RTE enables the users to use visual formatting aids to create bold, italic, paragraphs, tables, etc.

The rtehtmlarea RTE activated in the TYPO3 backend

For full details about setting up a field to use an RTE, please refer to the chapter labeled 'special-configuration-options' in older versions of the TCA Reference.

The short story is that it's enough to set the key enableRichtext to true.

'poem' => array(
    'exclude' => 0,
    'label' => 'LLL:EXT:examples/locallang_db.xlf:tx_examples_haiku.poem',
    'config' => array(
        'type' => 'text',
        'cols' => 40,
        'rows' => 6,
        'enableRichtext' => true

This works for FlexForms too: