An alert is a message that informs people of something. In SharePoint, we also need alerts to notify users of changes. Such changes are always related to a list or document library. To receive notifications about them, use the Virto Alerts & Reminder App to create an automatic email alert.
Example
You have a SharePoint list of team members’ personal information (age, education, marital status, address, etc.). Users can edit their data directly in the list. You set up an alert to notify the HR manager when changes are made to the list. For instance, Rachel Evans got married and changed her name. The HR manager will receive an alert about that fact.
To summarize, an alert is an email notification about changes in a SharePoint list or document library.