Adding a "title" field to video thumbnails in the Video Module
You can use hook_form_alter(), there is no any built-in option to do that.

You can also use the Field collection module to create a new field collection with a text field, and the video field.


  Provides a field-collection field, to which any number of fields can be attached.
  
  A field collection is internally represented as an entity, which is embedded in the host entity. Thus, if desired field collections may be viewed and edited separately too.