What does not injective mean?

Could you please explain to me what is meant by the term "not injective"? I'm having some difficulty grasping the concept, and I'd appreciate a clear and concise explanation. Could you possibly provide an example or two to illustrate the concept? Additionally, how does the lack of injectivity affect functions in general? I'm particularly interested in understanding its implications in the context of cryptocurrency and finance. Thank you for your assistance.