The subject of many poems is poetry itself, and here is a great example: Wordsworth's sonnet on the sonnet, which dwells on the paradox — dear to poets' hearts — of freedom within, despite, or even because of confinement:
Nuns fret not at their convent’s narrow room;
And hermits are contented with their cells;
And students with their pensive citadels;
Maids at the wheel, the weaver at his loom,
Sit blithe and happy; bees that soar for bloom,
High as the highest Peak of Furness-fells,
Will murmur by the hour in foxglove bells:
In truth the prison, into which we doom
Ourselves, no prison is: and hence for me,
In sundry moods, ’twas pastime to be bound
Within the Sonnet’s scanty plot of ground;
Pleased if some Souls (for such there needs must be)
Who have felt the weight of too much liberty,
Should find brief solace there, as I have found.
— William Wordsworth