Shrines are almost like fancy decorations that can allow you to level
your dragon past level 10. When you first get 50 level 10 dragons of one element, you are allowed to level that dragon and its hybrids to as high as level 15. Then when you get 50 level 15 dragons, you can now level your dragon of that element and its hybrids to level 20.
Bronze: Dragons up to level 10
Silver: Dragons up to level 15
Gold: Dragons up to level 20
NOTE: You can sell a dragon after you have leveled it up to level 10 and it will still count as a level 10 dragon. And the same thing with level 15 dragons.