Andreessen defines product/market fit:

"The customers are buying the product just as fast as you can make it -- or usage is growing just as fast as you can add more servers. Money from customers is piling up in your company checking account. You're hiring sales and customer support staff as fast as you can."

you have reached product/market fit when you are overwhelmed with usage—usually to the point where you can’t even make major changes to your product because you are swamped just keeping it up and running.

What I look for is a frantic founding team trying to deal with ever-growing numbers of happy, loyal, and ideally paying customers. Until then, stay lean, keep burn low, and resemble a Navy SEAL team instead of an Army battalion.

Finding product market fit = focusing on the market first

The problem, i.e. the market, is the real opportunity.

You need to find problems so dire that users are willing try half-baked, v1, imperfect solutions.

to find product market fit, choose a market where users have a real, meaningful problem, launch quickly, and listen to your users.