Type any text and watch it get smaller: common letters get short codes, rare ones get long codes, and the result is the exact same idea behind ZIP, PNG and MP3. The tree is the codebook, drawn for you.
ASCII gives every character 8 bits whether it appears once or a hundred times. Huffman instead builds a tree where the most frequent symbol sits closest to the root (shortest code), and no code is a prefix of another — so the bit stream can be read back unambiguously. The saving shown is the theoretical floor for this kind of symbol code.