The Mood Board is a collaborative practice to take time to discover and share inspiration for product solutions, features, and/or design elements. Team members take time to go search for ideas of what a solution might look like, then share those ideas and what they like about each one with the rest of the team.
Mood Boards are a great way to find inspiration for your product. They can be used to find existing solutions to problems you're aiming to solve or specific features or design elements you think would be a great fit for your product. In giving all team members time to go out and explore existing solutions, it helps provide creativity and direction for the team, as well as specific design elements for your Product Designers to consider when either building a rapid prototype or new A/B test. Sharing ideas in this way helps speed up the process of what your solution should look like, decreasing the time it takes to design and test new ideas.
Check out these great links which can help you dive a little deeper into running the Mood Board practice with your team, customers or stakeholders.