Aplane flying against a jet stream will travel faster than a plane traveling with a jet stream.

Like every other case of motion, it depends on your reference.

Relative to the air, the plane's "air speed" doesn't change, no matter
whether it's flying against a jet stream, with it, or across it.

But relative to the ground, the plane's "ground speed" is affected by
the speed of the air it's flying through.

(And that speed of the air is relative to the ground. It's called "wind".)