By one definition, to understand means to perceive the meaning of words. Words form the foundation of language and language is a lever for conveying ideas. This is why it is important to have a large vocabulary: you cannot reason about ideas you cannot describe.

“Give me a lever long enough and a fulcrum on which to place it, and I shall move the world.” — Archimedes

Learning new words feels like work, but they extend your language lever. A bigger lever can move bigger ideas that let you forget a thousand little ones. Like gravity, for example. You can either remember that teapots fall, freshly-poured beer falls, Macbooks falls, and so on. Or you can learn Newton's definition of gravity:

Gravity is an invisible force that attracts all objects with mass toward each other with a magnitude that is directly proportional to the product of their masses, and inversely proportional to the square of the distance between their centers.

That's a lengthy definition, but if you can parse it, you can forget a thousand little ideas and begin to understand the universe. Sometimes a heuristic is good enough, such as "things with mass accelerate toward the earth", but not if you want to conquer space flight.

So by knowing more words, we can afford to remember less and reason more keenly about fewer, but bigger ideas.