When I say talking to customers, I mean actually talking to the people who will use your products. At a big company, there'll be people in between you and the customer - salespeople, marketers, user research people, all of whom are proxying the customer to you. But unless you talk to a real customer, you don't really know the problem they're facing.
When you're building an MVP, it's very easy for product managers, engineers, and designers to think 'We know what we're building.' And almost always, we end up building something that we think is perfect but no one actually wants. Therefore, my number one rule is to not go too long without talking to customers.
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