What does it mean when they say the universe is expanding?

Evidence of the Big Bang Theory, Proofs & Expansion - Lesson

Universe Expanding Faster Than We Thought — a 'Crisis in Cosmology

Cosmic expansion is a given. Who inherits the cosmos is not

Expansion of the universe - Wikipedia

Is Universe Expanding?

The Beginning to the End of the Universe: The Big Crunch vs. The Big Freeze

What Actually Expands In An Expanding Universe?

Measuring the Universe's Expansion Rate

If The Big Bang Wasn't The Beginning, What Was It?

What does it mean to say that the universe is 'expanding' when there is nothing for it 'expand' into? - Quora

What is the universe expanding into?

Ask Ethan: What is the Universe expanding into? - Big Think