PowerPoint Online is a browser-based version of Microsoft PowerPoint, the ubiquitous slideshow app you've likely been using for years. PowerPoint Online is a "lite" version of the full-fledged desktop app and provides fast opening times for reading and editing .pptx and .ppt files. PowerPoint Online is great for reading PowerPoint files, making simple edits, and presenting your slides directly from almost any browser on pretty much any device.
Most PowerPoint files will open directly in PowerPoint Online when using SharePoint Online, OneDrive for Business, MS Teams, Yammer, and some other Office 365 apps. Files open in reading mode. To edit the files, simply click the "Edit Presentation" button and choose to edit the file in PowerPoint Online (directly in the browser) or PowerPoint (the desktop app). To present a file, click the "Start Slideshow" button.
Functionality in PowerPoint Online is generally limited to day-to-day editing, text formatting, simple tables, simple animations, slide layout changes, hyperlinks, and comments, though it provides excellent support for co-authoring (simultaneous editing by multiple users). All changes made in PowerPoint Online are saved automatically; there is no save button.
You'll want to avoid more complex tasks like some animations, editing headers and footers, adding Excel charts, inserting shapes and charts, and applying rich formatting (among others) in PowerPoint Online. Some animations are simplified and not as rich if you present the file in PowerPoint Online.
If PowerPoint Online doesn't provide the features you need to do the job, simply click the "Edit in PowerPoint" button in the menu bar to access the desktop version of the app and its more complex features. Some versions of the desktop app save updates automatically, but some do not. Be aware of how this feature works on your version.