Low-fidelity prototypes are a mock-up created towards the beginning of the product development life cycle. They are used to illustrate roughly how a website or app will work to address a customer need. At this stage, the specifics of how the product will operate are not necessary.
High-fidelity prototypes are used to validate the solution before it reaches the customer. They contain the fine details, and are created towards the end of the product life cycle.
It is important to mention that while these two have clear differences, none will be correct if you don't perform an accurate gathering of product requirements.