A proof of concept (POC) involves building a simple version of a product idea to test it with users before fully developing it. A POC should be completed in 1-4 weeks with a small team and focus on core functionality rather than polish. Usability testing the POC with real users provides critical feedback on whether the idea is worth pursuing further. For example, a POC for a stock trading app may include basic login, search, portfolio views, and simulated trading recommendations to get early feedback from potential users.