SharePoint Online is part of Office 365 and enables the creation of websites as well as a secure place to store, organize, and share information, accessible from any device.
The tool is primarily intended for collaborative work on files and sharing files with other participants, unlike OneDrive for Business (https://login.microsoftonline.com/?whr=almamater.si), which is mainly intended for file storage and sharing files with others, but not so much for collaborative editing of those files.
To access the tool, sign in at https://office.com/ (with your digital identity) and select SharePoint from the tools in the upper-left corner.