SharePoint Online is part of Office 365 and enables the creation of websites, while also providing a secure place for storing, organizing and sharing information, with access 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 primarily intended for storing files and sharing them with others, rather than collaborative work on those files.
To access the tool, sign in at https://office.com/ (with your digital identity) and select SharePoint from the tools in the top right corner.