What is Managed Code?

Managed code run under the control of CLR. So CLR execute this managed code.

All IL codes are managed code hence it is run under the control of CLR. And if have used any component which is not built on .NET framework then such referenced codes are called unmanaged code.

What is Common Language Runtime (CLR) in .NET?

Common Language Runtime or CLR is responsible for executing your application code and the code running under the control of CLR is also called managed code.

Any application written in any language of .Net framework will never directly compiled to machine understood code instead the compiler converts the code into a special language called MSIL (Microsoft Intermediate Language) and then CLR compile MSIL into platform-specific code.
So only the methods that are actually called during execution are compiled.

So in short, the .NET Framework understands only one language that is MSIL.

How to rename database table column with new name?

You need to use the sp_rename command, or use Management Studio to do it visually. Now, to change the Column Name from "Old_Name" to "New_NameChange" we can use command:

sp_RENAME 'Table_Name.[Old_Name]', '[New_NameChange]' , 'COLUMN'