The term "jig" can have different meanings in different tool discipline contexts. In the fabrication world, it most commonly indicates an assembly meant to hold parts in precise alignment for final welding. In woodworking, however, it can indicate any arrangement of components meant to simplify a task or procedure. Here are a few good examples of the latter sort:

MAKING STRAIGHT CUTS WITH A HOMEMADE JIG by HomeRemodelWorkshop


Woodworking- homemade kreg jig! Pocket hole jig! by izzy swan


How to Cut a Circle with a Jigsaw by lostinthekerf


Small shop saves money with homemade jigs by FDMC


Homemade Table Sander by Doug Smith


DIY pocket hole jig by Justin Levine