Think of chemical elements as nature's alphabet. Just as 26 letters combine to create every word in the English language, 118 chemical elements combine to create every substance in the universe. An element is the simplest form of matter — pure gold is nothing but gold atoms, pure oxygen is nothing but oxygen atoms. You cannot break gold down into something simpler using chemistry; it is already as fundamental as it gets.
What makes each element unique is the number of protons in its atoms' nuclei. Hydrogen has 1 proton, helium has 2, carbon has 6, gold has 79, and so on up to oganesson with 118. Change the proton count, and you change the element entirely. This is why the ancient alchemists' dream of turning lead (82 protons) into gold (79 protons) was impossible with chemistry — though modern nuclear physics can technically do it, the cost would far exceed the gold's value.
Of the 118 known elements, about 90 occur naturally on Earth. Some are spectacularly abundant — oxygen makes up 46% of Earth's crust by mass, and hydrogen is the most common element in the universe. Others are astonishingly rare: astatine is so scarce that at any given moment, there are estimated to be fewer than 30 grams of it in the entire Earth's crust. Elements heavier than uranium (element 92) are all synthetic, created in laboratories and particle accelerators, often existing for mere fractions of a second before decaying.