Why freemium matters to a first-time founder
Freemium sets how many people try your product and how many you must serve without pay. Usually only a small share of free users ever upgrade, so the free tier has to be cheap to run and good enough that people stay and recommend you. Draw the line in the wrong place and you either give away the job customers would pay for, or offer so little that nobody stays long enough to upgrade.
Examples of freemium
A freelancer can chase one invoice a month for free. Chasing every invoice, and seeing which clients always pay late, sits on the paid plan. That is freemium. The free version shows what the product does. The paid version is how the company keeps going.
Common misunderstanding
People treat freemium as a free trial. A trial ends. A freemium plan stays free. If the free version already solves the whole problem, almost nobody pays. If it solves nothing, nobody stays long enough to consider paying.
When to use it
Consider freemium when the product spreads because people use it, and you can draw a clear line between casual use and use someone will pay for. Skip it when free users cost more to serve than they will ever be worth.