The software doesn't add any limitations so it just depends on the images and designs you put into it or the fonts you use with it.
In the word stencil maker, the license of the chosen fonts is shown below the preview. Almost all the fonts on the site are SIL licensed, which should be ok for almost anything you'd want to to do, such as share a design on your own website. For details on the SIL license see here.
If using the photo-to-stencil tool, you’re probably limited by the copyright or trademark of the image you put into it. So you'll probably want to stick to public domain or creative commons commercial licensed photos or to photos you've taken unless it's just for personal non-commercial use.