During the reflection phase you obtain feedback from the users of your product through, for instance, user tests. Furthermore, this phase offers you the opportunity to reflect on the previous phases. Did I fulfil the desires and demands of the consumer(s)? Does the product achieve the design qualities? Which extra things do I need to discover to improve the product? During the reflection phase you write a plan in which you describe how you are going to start the next design cycle.
Keep in mind
i. Do not see it as a failure if users do not like and/or understand your product or service. This is a learning moment and a possibility for you to use their feedback to improve your product.
ii. You do not require a fully finished product to start the reflection phase. Sometimes a cardboard prototype or a video of an impression can be enough. Define for yourself exactly what the objectives within this phase are, and find a (minimal) demonstrator to achieve these objectives.