You start building your startup from business strategy. For example - you have an idea for an app - what's next?
You should find tech partners, start building your team. You have to conduct the market research and define what problem can you solve for people, what are your possible competitors, what can you offer to your users that others can't. Then you start developing an MVP - Minimum Viable product. (Read this article to learn more about Creating an MVP)
An MVP - is the first version of your product, validation of your business plan, that you can represent to investors. A good MVP is essential for successful fundraising.
On other hand, maybe you already have enough resources to grow your project. Then MVP is your opportunity to test market fit and user adoption. This is a basis for your further app development. You can change it, remove and add some features and develop your business strategy.