OneDrive is Microsoft's cloud storage service. Up to 5GB is free, 100GB is $1.99 a month, and Office 365 subscribers ($9.99 a month) get 1TB. Nowadays, Windows and Office default to trying to save files to it rather than locally, which is why it asks you to login to your Microsoft/MSN/Live ID account. You can sychronize/backup files to it but the company warns synchronizing over 100,000 files will impact performance. It was renamed from SkyDrive 10 years ago after the UK television channel Sky sued them.
Competitor Google Drive offers 15GB for free (which is also shared with your Gmail and Google Photos storage) and while they also offer 100GB for $1.99 a month, there is also no limit to the number of Gmail accounts you can make, each of which gets 15GB for free. It was originally called Google Docs (and Google Photos used to be Picasa)
Which of the two is more convenient depends on whether you use MS Office or Google Workspace more.
The Dropbox free account only gives you 2GB of storage + like Google Drive, $9.99 a month gets you 2TB there.
Think of these like a self-storage facility, only for data. And you get to rent a small locker for free.