Up to two captions can be added to the print layout by selecting the "captions" checkbox in the preferences dialog:
Then press the "Settings" button to display the dialog below to setup the captions:
Each caption can occupy more than one line if required and is left justified by default. Select the "Center justify" checkbox to center justify multiple lines. The size, font, color and rotation (in degrees clockwise) of the caption text can also be specified. The font size is calculated using the printer resolution and so a 72 point font should give text one inch high (one 'point' is 1/72 of an inch). The caption is printed in the selected font color with a transparent background. Tokens can be used in the caption text to insert values such as %d for the date and %t for the time the first image in the layout was taken or {filename} for the filename used for the JPEG copy of the print layout. Please see the section on tokens for a list of available tokens.
Finally enter the position for the captions as offsets in mm from the top and the left of the page.