What is the attribute in the C#.NET code?

Say for example below,

using System.ServiceModel;

[ServiceContract]
public interface ICalculator
{
[OperationContract]
// It would be equivalent to write explicitly:
// [OperationContract(IsOneWay=false)]
int Add(int a, int b);

[OperationContract]
int Subtract(int a, int b);

int Multiply(int a, int b)
}

I do not know what the attributes means and uses for like [ServiceContract] or [OperationContract].

Thank you for replying.

Kin Liverpool