Captions will only show if your image has a caption added.

screenshot showing featured image captions with the Definite theme
This shows only if a featured image has a caption added

However, if you do not want them to show, follow this tutorial:

  1. Go to Appearance > Customize > Theme Options > Blog Settings.
  2. Look for the Hide Post Summary Featured Image Caption setting and check the box to disable captions.
  3. For pages, go to Theme Options > Page Settings
  4. Look for the Hide Page Featured Image Caption setting and check the box to disable captions.
  5. For the full post views, go to Theme Options > Post Settings
  6. Look for the Hide Full Post Featured Image Caption setting and check the box next to that one.
  7. Click the Publish button.