Definition of Database Maintenance Plan:
A database maintenance plan is a set of specific, proactive tasks that need to be performed regularly on databases to ensure their adequate performance and availability.
Maintenance plans are essential. Microsoft provides two tools, the Maintenance Plan Wizard and the Maintenance Plan designer, to assist the busy DBA to get started with one. But they do not include all the possible tasks, so Custom-created T-SQL or PowerShell scripts are usually required sooner or later to provide a more comprehensive plan.
The following article may be a good reference for companies wanting to setup a good database maintenance plan for their SQL systems: