what is meaning of this line of code:--while ((sentence = Console.ReadLine()) != "[x]")

using System;
using System.IO;
using NFastTag.PosTag;

namespace NFastTag
{
   class Program
   {
      static void Main(string[] args)
      {
         Console.WriteLine(@"Welcome to FastTag_v2.NETPORT");
         Console.WriteLine(@"Enter an english sentence and watch it being tag");

         // read the english lexicon data
         var lexicon = File.ReadAllText("Grammar\\lexicon.txt");

         // run the sample loop
         var ft = new FastTag(lexicon);

         string sentence;

         while ((sentence = Console.ReadLine()) != "[x]")
         {
            var tagResult = ft.Tag(sentence);

            foreach (var ftr in tagResult)
            {
               var message = string.Format(@"[{0} {1}]", ftr.Word, ftr.PosTag);
               Console.WriteLine(message);
            }
         }

         Console.WriteLine(@"Bye Bye!");
      }
   }
}