Coleridge briefly includes certain images from nature: frost and a crying owl at the beginning, and later the sky, stars, lakes, mountains, a robin in spring… and, yes, more frost. He explains that these are all part of God's "eternal language"—Nature is a form of speech which God uses to address humankind. But what is God saying through Nature? Apparently, he's announcing his own infinite creative power and his own existence. By giving this gift to humanity, this revelation, he's teaching people how to search for him and try to know him.
Lines 52-59: Here, Coleridge is describing how his son will wander through all this natural imagery, happily finding some kind of meaning in it. Like the clouds reflecting shapes on earth, all of Nature seems to be reflecting a higher truth.
Lines 59-65: This higher truth is revealed in these lines: God uses Nature as a language, to speak to humans and get them to develop a relationship with God, asking him for help and wisdom. God is a teacher and Nature is the lesson he teaches. (The final lines of the poem, from 66-75, also seem to buffer this point, though less directly.)