In short spell jars are whatever you want to make them for. You can make one for protection, love, prosperity, or many others.
You take a small jar with a cork lid and fill it with ingredients that match your intentions. For example; if you are doing a spell for prosperity you might seal it with a green candle to represent abundance etc.
Types of things you can put in a spell jar to map out your intentions; herbs, oil, honey, piece of paper with intentions on it, money, crystals, seeds, fabric, small keepsakes, anything that you can stick in the bottle to represent your intentions will strengthen your spell.
Once you figure out your ingredients you can burn incense and let the smoke cleanse the jar. Then add the ingredients and seal it shut with candle wax. Say your intentions out loud and close your circle.
Some people prefer to store this upside-down underground afterward. I live in a place that is not always possible so, I just store them safely on a shelf.