Blake is expressing a mystical belief that the microcosm symbolizes the macrocosm and that it's possible to experience the macrocosm by meditating on the microcosm.
When you look at a grain of sand correctly, what you really see is the entire world in a mystical vision of itself. When you view a wild flower with your whole self present, with all your senses, you really do see heaven.
From this perspective, the palm of your hand is as large as infinity and an hour is as long as eternity.
I remember a biblical verse that goes something like this " A day in the year of the life of the lord is as a thousand years". I think there is some wisdom contained in this scripture. The roman's couldn't hide it all.
Image by Mezza - White roses signify Peace