Intellisense is a new feature that comes with SSMS(SQL Server Management Studio) 2008 onwards. Intellisense makes coding quite easy. Sometimes I find that intellisense becomes stale and you start getting the wavy red lines when writing code. Even though object exists in database but Intellisense is not able to recognize it.
Refreshing the cache is quite easy but not necessarily strictly required.
Go to Edit -> IntelliSense -> Refresh Local Cache OR you can use shortcut Ctrl + Shift + R