Full many a gem of purest ray serene, The dark unfathomed caves of ocean bear?

Question

can someone translate this for me, please

Answer 1

It makes more sense if you study the next two lines as well:

'full many a flower is born to blush unseen

and waste its beauty on the desert air'

There are many things of beauty that remain hidden, or that live and die, unappreciated and unseen.

This could also refer to human thought and deed; many works go unappreciated, either those of poets, artists or even scientists.

Some see a deeper sense, that our 'purest ray serene' is life, and 'the dark unfathomed caves' are death.  This figures, as Gray wrote his famous elegy inspired by a graveyard.