Like the others were saying it’s a note taking tool, it stores notes in a markdown format so you can better own your notes because it’s not stored in some weird format and is just plain text.
It has a few extra features primarily from its community plugins but there’s also the graph view and more recently added, the canvas/whiteboard view. Where the former provides a graph of how your notes connect when you inter-link them like your own personal wiki; and the latter aims to provide a 2D visual board to put notes, links, and other content and connect them.
Also like the others said backup/sync is a paid feature, but there are a few quite simple ways to sync with free services such as Google Drive, iCloud, Dropbox, or Syncthing (device to device sync)
Wdym? Do they not offer the parts online or something?