Christine Hawthorne has a great post about microcopy on the GatherContent blog.
User experience design aims to make things feel intuitive for the person using your app or platform. Microcopy needs to act in the same way.
Just a few, carefully chosen words can go a long way in apps and can stop users struggling or dropping out of the process altogether.
Microcopy shouldn’t explain the design. It should enhance the user experience, working within context and to answer the question a user might have. For example, the copy on a button shouldn’t tell users to click it. It should say where they will go next, or what will happen when they press it, i.e, it saves the information.