SharePoint is a Microsoft platform offering collaboration, document management and process management functionality by means of web portals, by providing a centralized repository for shared documents and processes, as well as browser-based management and administration of them.
SharePoint offering as a product (Microsoft SharePoint Server) which installed on premised and as a service through Office 365 (SharePoint Online).
The biggest advantage of a SharePoint is that everything can be controlled by a workflow and the end user can be informed and manage the information by a single location. For example, we have a process that we need to monitor the contracts of our company, Contracts have documents, dates, noted and many stakeholders. With SharePoint we can have the Contract as an entity, living in a SharePoint list with information like, customer, start date, end date, sign date, notify date, stakeholders and the files. Also we can have a workflow that will get approval by the owner of the contract, inform the stakeholders for the expiration X day before the expiration and also the Stakeholders can coauthor at the documents.
In two word, SharePoint is a Collaboration platform.