To see a World in a Grain of Sand And a Heaven in a Wild Flower, Hold Infinity in the palm of your hand And Eternity in an hour. (1-4)
These lines provide the key to the rest of the poem. Blake wants you to take its advice, and look for big realities hidden in small images.
The wild deer, wand'ring here & there, Keeps the Human Soul from Care. (21-22)
A carefree human soul is kind of like a wild deer—since the deer doesn't have any greater obligations, it's free to go where it wants to go. Unlike the robin in a cage at the beginning of the poem, the deer is an image of freedom.
Joy & Woe are woven fine, A Clothing for the Soul divine; Under every grief & pine Runs a joy with silken twine. (59-62)
Joy and woe are like clothing for the soul because they're experiences that it puts on when it enters the world. Joy is made of "silken twine" because it's like the fine Under Armor that lies beneath a coarse layer of outerwear (woe).
To be in a Passion you Good may do, But no Good if a Passion is in you. (111-112)
Blake doesn't want people to be driven by passion—he wants them to drive it, and to stay in control. You should use passion, but not be used by it.
Every Night & every Morn Some to Misery are Born. Every Morn & every Night Some are Born to sweet Delight. Some are Born to sweet Delight, Some are born to Endless Night. (119-124)
People are passing through different states all the time in this world, says Blake. They could be headed from joy back to suffering, or through suffering back to joy. (He might be implying that joy will only be permanent in eternity.)