TBH stands for "To Be Honest" and is used to express honesty or sincerity in a statement. IDK stands for "I Dont Know" and is used to indicate that the speaker does not have an answer to a question or does not know how to respond. These acronyms are commonly used in text messaging and online conversations. While TBH is used to express honesty, IDK is used to express a lack of knowledge or uncertainty in a conversation.