Product-Market Fit
Product-market fit is one of the hardest things to get right, and without it, your startup will eventually fail. So you've got a potential product to solve a problem, and you've got a potential market; now it's time to find the product-market fit. Let's say that your startup is focused on providing radiologists with a tool that helps them to quickly identify certain skeletal conditions at-scale with artificial intelligence using machine vision.
When thinking of the market, you need to narrow in on a specific subset of the market that has the strongest need, or has already indicated (ideally with pre-payments) that they're willing to pay for a solution to their problem. You might first start off by saying that the market is 'Medical', but you'll need to keep breaking this down into smaller subsets (similar to the way that you'll take a large problem and break it into smaller, more manageable pieces). An example of a subset to the 'Medical' market would be 'Medical Imaging', then potentially focusing on a specific type of skeletal expertise (i.e. your product is really good at identifying skull trauma, etc.). The smaller the subset, the more you carve out a niche. This is an excellent way to get your initial product sales off the ground.
Despite identifying a subset in the market, when you speak with customers, you'll likely notice that their needs vary slightly. Your goal here is to make sure that as you develop your minimum viable product, you're only adding value to the most common pain points. In doing this, you'll develop a market segment with clear value-adds for the niche you've selected, without your MVP turning into a bloated product that has a ton of bells and whistles. If you added everything that each customer suggested, you'd be shorter on time to develop all of the features, and your product would likely become confusing and full of unnecessary features. Your job as a founder is to listen, categorize, plan, and execute in a way that minimizes time and maximizes value for your customer. You'll know if you have a market segment when you're able to sell the existing product to customers in your niche without having to change the product.
Once you've established a market segment, you will start to become a market leader in that niche. To achieve product-market fit, you'll now need to look at other market segments that have similar pain points and address the segments that require the least amount of change to your existing product. Over time you will start to gather a group of segments within the overall market, and you'll start to notice that you're addressing their problems / needs so well to the point that your customers are coming to you. This is product-market fit. Once product-market fit is achieved, your next step is to gobble up as much market share as you can, as quickly as you can.