Seamless patterns are images, which you can place alongside each other. It comes along with copies of itself without any interruptions and visible seams in its content. Making an image seamless is beneficial in cases that you wish to repeat the image in order to create infinitive patterns.
Open the chosen image first to create a seamless pattern in Photoshop. Choose Filter, then Offset, finally to the Original Image. To start with the process, increase first its values to easily see the offset for the Unidentified area. Finally, choose to wrap around. You can start making your image seamless.