what is using static in c#

In general, static means “associated with the class, not an instance”.
// Search c# static review for more detail

In C#, static means something which cannot be instantiated. You cannot create an object of a static class and cannot access static members using an object. C# classes, variables, methods, properties, operators, events, and constructors can be defined as static using the static modifier keyword.

class SimpleClass
{
    // Static variable that must be initialized at run time.
    static readonly long baseline;

    // Static constructor is called at most one time, before any
    // instance constructor is invoked or member is accessed.
    static SimpleClass()
    {
        baseline = DateTime.Now.Ticks;
    }
}

// The using static directive designates a type whose static 
// members and nested types you can access without specifying a type name.
using System;
using static System.Math;

public class Circle
{
   public Circle(double radius) 
   {
      Radius = radius;
   }
   public double Radius { get; set; }
   public double Diameter 
   {
      get { return 2 * Radius; }
   }
   public double Circumference 
   {
      get { return 2 * Radius * PI; }
      // otherwise if not using static "get { return 2 * Radius * Math.PI; }"
   }
   public double Area 
   {
      get { return PI * Pow(Radius, 2); }
     // otherwise if not using static "get { return Math.PI * Math.Pow(Radius, 2); }"
   }
}