All the patterns on this site are seamless. What is a seamless pattern you ask?
It is a repeating image (usually, small in file size and dimension) that does not have seams or indentations so that when tiled (usually, both horizontally and vertically) it gives the appearance of a single coherent image. These patterns are useful in web design, graphic design, mobile app development and other related fields due to there simplistic repeating nature. Sometimes they are even especially necessary due to the image files not being large or clunky and can improve user experience, inter-activeness, and even SEO.
The patterns on this site are licensed under the CC0 and as such can be used for free, in commercial projects, and even without linking to this site (though I would really appreciate it if you did).