The term “behold” functions as a finger pointing to the horse. John wants his readers, beset with the struggles of life in this fallen world (and detailed in earlier chapters, including the questions relating to the beasts and the dragon), to take careful note of this horse and be encouraged.
11 Then I saw heaven opened, and behold, a white horse! The one sitting on it is called Faithful and True, and in righteousness he judges and makes war.