What is an example of not injective?

Could you please explain what is meant by "not injective" with a concrete example? I'm having a bit of trouble grasping the concept. Could you illustrate it with a simple scenario or mathematical function that is not injective? It would be helpful if you could also explain why it is not injective and how it differs from an injective function. I'm particularly interested in understanding the consequences of a function not being injective and how it affects the mapping between input and output values. Thank you in advance for your clarification.