"Hanja(한자, 漢字)" refers to Chinese characters that are used in Korean, and even if you don't actually know how to read and write the Hanja characters themselves, knowing Hanja-based words will exponentially increase your Korean vocabulary since many Korean words are based on these Chinese characters.
-Do Korean people really know thousands of Hanja?
Many of them do! And most people can at least read and understand hundreds of Chinese characters but they may not be able to write everything from memory.
-Will knowing these Hanja characters actually help with my Korean vocabulary?
Definitely! Learning Hanja can be compared to learning Latin and Greek word roots when learning English, but much easier and much more applicable to everyday usages.
- So, even if I don't actually memorize Hanja characters, studying Hanja can still help me improve my Korean?
Yes! Studying Hanja will help you develop a sense of understanding and even guessing the meaning of new words you encounter for the first time!