Day one of the design sprint is about gathering all existing information/knowledge on the business, the customer and the problem and exposing our assumptions and knowledge gaps. From here can make plans to fill the riskiest knowledge gaps and validate or invalidate our riskiest assumptions.
Throughout Understand you should be working on defining the business, who is going to be using the product, how this product is solving a problem that those users have. What are the situations that users will be using the product or feature? What are their motivations behind using it? What are any outside motivators that might affect their use? The whole team should think about what product success looks like and what success looks like for just the sprint. The team should also talk about what they don’t know, where knowledge gaps are for the problem. As all of those are being defined assumptions that. The team should cover existing research done before the sprint and review analysis of competitive products.
At the end of Understand the team should have captured the critical path for the problem.