It is the inner vision that is opened or opening in you. When that opens, the first thing that you see is colours or lights moving or small or vague shapes or objects —afterwards flowers etc., then figures of people, scenes, landscapes, things happening etc. Often by the power of this subtle vision the sadhak can see the image of the Divine he worships in his heart and so feel more concretely the presence.
Ref: Letters on Yoga — III