It’s important to de-bubble jars after filling them and before you put the lids on, to make sure there is no trapped air in the jar. (See the page on headspace to understand why.)
You use something to poke the each side of the jar, pressing the contents gently towards the centre, then do the centre a few times if the type of contents allows. After this, often enough, the fill level will have dropped visibly owing to the air that escaped. You then top the jar back up with liquid or food to the recommended headspace for that food product.
Whatever tool you use to do this is your “de-bubbler” (aka “air bubble poker”, “bubble remover”, etc.)
You can use something like a a wooden chopstick or a small plastic spatula, or, you can buy a specially designed long, thin plastic sort of blade for it.
The one thing that is definitely recommended against is a metal knife. The reason given is that over time, a metal blade will produce imperceptible nicks and scratches in the glass that could weaken the jar. ( Mind you, it would be a fair question to ask what’s to prevent people in the household doing just exactly that with the jar when it’s opened and in use for ketchup, relish, etc.)
You can also buy combo devices. These devices are a de-bubbler poker on one end, and a headspace measurer on the other. But if you’ve been sticking the pokey end in a nice hot sticky relish, why do you want to turn that end around into the palm of your hand as you measure headspace? But given that they are so inexpensive, perhaps it’s not a worry to just have two on hand.