What is injective explained?

Could you please elaborate on the concept of "injective"? I'm trying to grasp its essence but am finding it a bit challenging. Could you provide a simplified explanation of what it means and how it's used in the context of mathematics and potentially other fields like cryptography or finance? Also, could you give some examples to illustrate the concept? It would be greatly appreciated if you could keep the explanation concise yet comprehensive. Thank you in advance for your assistance!