It can target any number of images in a web page that has a specific
class and apply the watermark over the original image with the help of HTML5 canvas.
The library normally aims to protect images from being stolen. Although, it is not a perfect solution (just like many others) as the image source is plain-coded, it can still prevent non-advanced users.
On the other hand, watermark.js is very handy when you only want to insert an image over another and position however you want without a server-side solution.