Pictures are often better than words to describe thoughts, feelings. On a Moodboard, you gather images but also texts, colors that will help you communicate about the overall feeling, the style of the product/website you are creating.
Moodboards are used in many different creative areas such as photography, fashion, interior design... They can be used in 3 main ways: to present your idea to the client, in a co-creative way with the client or stakeholders by creating one altogether, to communicate in between the team. The goal is to keep it quite broad, not finished to avoid confusing the client, who by the way should be explained clearly about the role of the moodboard.
Usually a moodboard is a one page document, digital or physical. To me, a big physical board is the best to communicate. You can choose between different techniques from a collage to a really structured page. But again, it shouldn't look like a finished document.
The possibilities behind moodboards are large, you can choose to focus only on one part of your project, create them in many different ways from a pinterest board to a big collage cardboard, with materials to show textures... but the best is to use and show them early on the process to ensure everyone is on the same line.
Moodboard example by, collage style.