What does static mean

Question

Hi

public static class CommonFunction
{
   public static bool IsAlphaNumericWithUnderscore(string input)
   {
     return Regex.IsMatch(input, "^[a-zA-Z0-9_]+$");
   }
   public static bool IsAllLetters(string s)
   {
     foreach (char c in s)
     {
       if (!Char.IsLetter(c))
         return false;
       }
       return true;
   }
}

Thanks