Understanding the thing to build. Some useful reminders from Hiten:

When you only have a vague idea of the problem you’re trying to solve, don’t skip a step by thinking about the technology you could build. Identify the existing solutions that people in your space are using—then dig deeper into how they’re being used.
The biggest opportunities for your product rarely have to do with how much better, faster, or more innovative your product is. By researching and really understanding all of the different use cases for competing products, you start to figure out which customer needs the competition is failing to serve. 
— Hiten Shah