what is enum data type.. why we have need to use enum

The enum keyword is used to declare an enumeration, a distinct type that consists of a set of named constants called the enumerator list.

public class EnumTest
{
    enum Days { Sun, Mon, Tue, Wed, Thu, Fri, Sat };

    static void Main()
    {
        int x = (int)Days.Sun;
        int y = (int)Days.Fri;
        Console.WriteLine("Sun = {0}", x);
        Console.WriteLine("Fri = {0}", y);
    }
}

In the above example, enums can represent and use days directly.
As a result, there is no need to define days in a collection.

Source - enum (C# Reference)[^].