In most use cases, you have an uniform 2D Vector as UV input.
Think about the UV input as “how often does the Texture fit into the normalized UV-Space?”
The higher the numbers, the more often the texture repeats (= the texture gets smaller)
The smaller the numbers, the less the texture repeats (= the texture gets bigger)
U is horizontal repeating, V is vertical repeating
This can be useful when it comes to Landscape Materials. To avoid visible repeating, you scale down the texture by decreasing the UV Input.