An Alt text, or Alternative text, is a text associated with an image that describes what the image depicts. Search engines cannot read images and therefore look for the Alt text of each images.
In Portfoliobox, the Alt text of your image is the image title. The image title can be edited in each gallery. You can choose to display or hide the image title on your site under Display Settings — depending on the template you use, the image title will display under the image or on hover. In both case, the image title will be the Alt text.
NB! Keeping the field Image title empty might cause your images to not be indexed in search engines.