What is a webapp?

Examples of webapps

GMail was one of the first webapps to become popular, it mostly replaced desktop apps like Microsoft Outlook. GMail took off because you could view your emails on any device.
Figma is one of the fastest growing current webapps, it replaces several design tools like those by Adobe. Figma is popular because it's much easier to collaborate with versus local apps.
ClipChamp is an up-and-coming webapp, it's doing for video editing what Figma did for offline design tools.

There are three key components you need to choose when first building your webapp:

The marketing site: You can either program the site yourself, or use something like Wix, Squarespace, or Webflow.
The frontend: You'll need to choose a Javascript-based framework to build your actual website in. The most common choices here are React, Angular, and Vue.
The backend: To actually store data, you'll need to use a backend stack. If you're just starting off, this will likely be Firebase or Heroku.

If you're not sure where to start, the industry is currently favoring Webflow, React, and Node.js (Heroku) for the three categories.