Complex sentence: A Progressive Web App is an application that has been built using a set of web development techniques and technologies that help enable a web site to be progressively installed and act like a mobile application as the user becomes increasingly engaged with the experience.
Split into simpler sentences: A Progressive Web App is an application that has been built using a set of web development techniques and technologies that help enable a web site to be progressively improved depending on the user. For example a web application can be installed and act like a mobile application as the user becomes increasingly engaged with the experience.