Software that has a user interface appearing in a web browser. These applications may run on third-party servers or be self-hosted.
Web-Apps usually don't require you to install anything on your own device(s) other than a web browser – though sometimes you can do so (e.g. hosting them on your own server). Those applications are installed on a web server, where they can be used from anywhere – on the condition that an Internet connection is available. While the application itself runs on the server it is hosted on, the documents, images, or other data created with it are stored on, or can be downloaded to, your local device(s). Thus, in the ideal case, you have no maintenance tasks to perform (software updates are performed server-side).
Please do not confuse this with hosting questions. Your question should always be for the software – not for the place where it is installed (see: Are hosting recommendation requests on-topic?).