A pattern is an image that is repeated, or tiled, when you use it to fill a layer or selection. Photoshop comes with various preset patterns.
You can create new patterns and save them in libraries for use with different tools and commands. Preset patterns are displayed in pop‑up panels in the options bar for the Paint Bucket, Pattern Stamp, Healing Brush, and Patch tools, as well as in the Layer Style dialog box. You can change how patterns are displayed in the pop‑up panels by choosing a display option from the pop‑up panel menu. You can also manage pattern presets using the preset manager.
If you are using a pattern from one image and applying it to another, Photoshop converts the color mode.
Photoshop comes with a set of Illustrator format files that you can use to define a preset pattern. Open the file, select any rendering option, and then define the pattern.