In the words of Alex Russell, who coined the term Progressive Web Apps, PWAs are “responsive, connectivity-independent, app-like, fresh, safe, discoverable, re-engageable, installable, linkable web experiences.”
If that is a mouthful for you, I do have a simpler description:
PWAs are a happy medium between web pages and native mobile apps. They combine the best of the two to bring an enhanced user experience to the consumers.