one of the phrases that gets used most when talking about startups is product/market fit. countless blog posts, talks and books are written on the subject for good reason, it's pretty important.
if you don't have product market fit, you're going to fail, as you've just built something that no one wants. this can suck if you've spent a year working on it.
however, before product market fit, founders should consider whether what they're building is right for them. i think this is hugely important for solo founders/indie hackers.
product/founder fit is how well the product you're building is right for you.
for example: if you're building the next big social media marketing tool, are you vageuly interested in that market? can you relate to the customers that you'll be selling to? on a higher level, do you think the market, social media is a good thing, are you going to like working on this?
if you're a solo founder, you're going to end up working on this thing for next couple of years, so you've got to enjoy what you're doing on a daily basis. if you don't you're going to be in trouble. motiviating yourself to work on something is hard, it's even worse if you don't enjoy the market you're serving.
basically: if you don't like coffee, don't start the next starbucks.