The authors of the Romantic period were heavily influenced by their dislike of the preceding literary movement (The Age of Reason). Historically, any movement which exists comes about given a disagreement with the works of a current period. The Romantics did not like the generalizations and instructive aspects of the Realists. Therefore, the Romantics focused upon the importance of feelings, imagination, and intuition.
With this, the Romantics centred their writings around nature given, for many of the authors, nature evoked deep feelings which were perfect for the ways in which they wished to write while focusing on imagination and intuition.
To explicate an essay on the Romantics (for example), you could find different examples of authors and how they used nature to explore the abstract ideas of the imagination, feelings, and intuition.
A few good authors to look at in regards to these characteristics are: Emily Dickinson, Walt Whitman, William Blake, and Henry Wadsworth Longfellow.