In simple terms, Microsoft SharePoint is web-based document management and collaboration platform that can be customized according to requirements.
As part of document management, you can create, edit, store documents in your SharePoint application and as part of collaboration you can share document within your workspace. You can also apply security policies on your documents to ensure it is shared to specific user groups only.
There’re many uses of SharePoint but on high level, SharePoint is used as a document management and Collaboration platform.