This article describes all formats that use the PAT file extension (there are several of them) and explains how to open each type.
AutoCAD Download and Install New Hatch Patterns (PAT Files) Tutorial
A file with the PAT file extension is most likely a pattern image used by graphics programs to create a pattern or texture on an image using a small and usually square image.
If the file you have isn't in that format, it might be something else that uses the same PAT extension. For example, it might be a DiskStation Manager system file, Gravis UltraSound GF1 instrument patch file, Ketron sound pattern file, or Kega Fusion cheat code file.
Pattern images can be opened for free with GIMP and even online with Photopea. If you choose the latter, open the PAT file as you would any other file, via File > Open; open the patterns via Edit > Preset Manager > Pattern.