With this feature, you can create captions from the photos or videos you use in the Compose window.
To use this feature first open the Compose window with the 'Create' button.
After selecting 'Compose', the Compose modal should pop up and you'll be able to add an image by uploading an image or selecting from the gallery.
Once an image is selected, you can hover your mouse over the image to see the 'Auto-Caption' button in the center. Clicking the button will generate a caption based off of the image you selected.