It is a full fledged flight planning SW (with Lido charts for airports and airspace informations, etc, …) from which you can access either via the linked webpage or in game via the integrated EFB. The planning tool in MSFS2020 is very very rudementary in comparison to this one.
The webpage also integrates fully in the sim: you can plan your flight on any computer you have internet access and store the plan online → it is immediatly accessible in the sim via the EFB.
Edit, for more information you can see for example: MSFS 2024 Flight Planning and website