PMF (product-market fit) is the act of being open to constantly iterating changes to a product (in our case, an app) to ensure that it best resonates with its target audience.
PMF starts with the creation of a solid MVP, which is then released onto the market. As user feedback is gained, the product is modified to fall into line with what the market wants and expects, until a product-market fit is achieved. Sometimes this will result in a final product which is much different from the one originally launched on the market.
Notable companies which embraced the PMF concept are Airbnb and Instagram.