Building in public means sharing the making of a product as it happens: what you shipped, what broke, how many users and how much revenue you have.
It is usually done on X, LinkedIn, Indie Hackers or a blog, in short regular updates. The updates that travel are specific: a number, a screenshot, a decision and the reason for it.
The audience it builds is mostly other makers. That is useful for feedback and for launch day, less so if your customers are a different crowd.
Why it matters for a launch
People who have followed a product for months show up when it launches. It is the slow way to have an audience on launch day, and it costs nothing but consistency.
Read more: How to promote your app for free