A Microsoft file hosting and synchronization service.
Click on your Storage Meter in the bottom left corner in OneDrive on the web to open the Manage Storage section. Here you can click on "Get more storage" to add additional storage to your subscription.
This direct link should take you there as well; https://onedrive.live.com/?ocid=cmmti3rc7hf&v=upgrade
I'm not sure how often you can add additional storage to your subscription to go beyond the 2TB limit. However, you can then also consider obtaining a business license which can grant you up to 5TB or even 25TB (depending on the subscription).