Meaning: A cat jumped quickly and powerfully to a position on top of the ledge at the bottom of a window.
Choice of Words: The word "cat" refers to a small domesticated carnivorous mammal. "Leaps" means to jump suddenly and forcefully. "Onto" means to a position on top of something. "The" is used before "windowsill" to specify a particular windowsill. "Windowsill" refers to the horizontal ledge at the bottom of a window.