Select asset tiles like images, video, file, etc, and certain asset render, like when an image tile is expanded, allows captions to be added for it.
To add a caption:
Click the tile.
Click on +Add Caption
Enter your caption text.
Use the text editor to style the caption as needed.
Note: Pasting a caption from another application might include an HTML design or format that could overwrite the site - or theme-level designs or customizations. This causes inconsistencies with the design of the caption when rendered, such as with its line height, spaces, and gaps between lines, etc. To prevent these inconsistencies, copy the captions as text only. This feature on most browsers is found under the Edit menu.
For a video walkthrough of caption setup, see the guide to adding images and tiles with captions and descriptions.