On common benchmarks the best open-weight models now sit close to frontier commercial models, and for many routine tasks the difference is not noticeable. The remaining gaps show up in long-horizon reliability, tool use, very long contexts and safety tuning — and in the operational work of running them yourself.
Open weights means the trained model file can be downloaded and run yourself, under whatever licence the publisher chose. Open source is a stricter legal standard requiring freedom to use, study, modify and redistribute without restrictions on field of use. Many widely used models are open weights but not open source.