Theory:

From our dark spirits. Such the sun, the moon,
Trees old and young, sprouting a shady boon
For simple sheep; and such are daffodils
With the green world they live in; and clear rills
That for themselves a cooling covert make
'Gainst the hot season; the mid forest brake,
Rich with a sprinkling of fair musk-rose blooms:
And such too is the grandeur of the dooms
We have imagined for the mighty dead;
All lovely tales that we have heard or read:

An endless fountain of immortal drink,
Pouring unto us from the heaven's brink.
Explanation:
  
The poet says, the beauty around us comes in some form, to lift us from our darkness. Only beauty can save our spirit and soul, from the evils of life. Despite all the miseries of our life, beauty comes in the form of everyday things like:
  • Sun
  • Moon
  • Old trees or new plants - which extends its shade to us, as a blessing
  • Simple animals like sheep - the reference can also mean fellow men on this Earth. It is believed that Jesus was the shepherd who led men (also known as sheep)
  • Daffodils - beautiful flowers which grow in spring - they symbolize new beginnings
  • Clear streams of water that gives coolness even during hot summers
  • Thick, dense bushes in the middle of forests
  • Abundant growth of musk-rose blooms
beauty.jpg
The sun, moon, trees, fellow men, daffodils, clear rills, thick bushes, musk-roses - the various shapes of beauty!
  
The poet appreciates the beauty in all the above. He says nature has provided us with such beautiful things to lift our spirits and souls from the depressing and gloomy events around us. Not only the above, but he also finds similar beauty in the inspirational stories of the great men/kings who have done many noble deeds and left behind their legacies. He finds such stories beautiful and splendid as nature.
 
legacies.jpg
There is beauty in inspirational tales of the mighty noblemen!
 
Finally, the poet says nature provides us with abundant beauty that it looks like a never-ending fountain of a drink, which falls on us and lets us live forever. It is like a shower of blessings. The beauty of nature is like a nectar that flows straight from the edge of heaven. If we take inspiration from the beauty around us, we can live forever happily and healthily.
 
fountain.jpg
An endless fountain of immortal drink, from the heaven!
 
Meanings of difficult words:
  
No.
Words
Meanings
1
sproutinggrow out shoots
2
daffodilsa yellow, bell-shaped spring flower with a long stem
3
rillssmall streams
4
coverta dense, but generally small, growth of shrubs, bushes or small trees
5
musk-rose(of the Mediterranean region) white flowers having a musky odor
6
grandeursplendour and impressiveness
7
doomsdestruction, death or any unavoidable situation
8
immortalliving forever
9
brinkthe extreme edge of land before a steep slope or a body or water
Reference:
State Council of Educational Research and Training (2019). Term-2 English Standard-8. A thing of beauty - John Keats (pp. 127-130). Published by the Tamil Nadu Textbook and Educational Services Corporation.