A Summer Evening Churchyard
To understand "A Summer Evening Churchyard," it is helpful to consider both the poem's themes and Percy Bysshe Shelley's biographical context. 
Main Themes and Ideas:
Meditation on Death:
The poem uses the setting of a churchyard at twilight to contemplate mortality. However, Shelley's approach is not entirely morbid. He explores the idea of death as a form of peace and rest.  
The poem explores the concept of Death as a natural part of the cycle.
Nature and Tranquility:
Shelley's characteristic use of nature imagery is prominent. The evening setting, with its fading light and quiet atmosphere, creates a sense of tranquility.
The natural world serves as a backdrop for reflection, and the poem seamlessly blends observations of nature with introspective thoughts.
Romantic Sensibility:
The poem reflects the Romantic movement's emphasis on emotion, nature, and the individual's experience.
There's a sense of awe and wonder in the face of the natural world and the mysteries of existence.
The passage of time:
The setting of the Evening churchyard and the transition of the time of day lend themselves to meditations on the passage of time and its relation to human existence.
Percy Bysshe Shelley's Biography:
Shelley was a significant figure in the Romantic movement, known for his radical political and social views, as well as his lyrical and imaginative poetry.  
His life was marked by unconventional behavior, including his expulsion from Oxford University and his controversial personal relationships.  
He had a deep connection with nature, which is evident in his poetry.  
Shelley's philosophical and political beliefs often influenced his writing, and his works frequently explore themes of freedom, justice, and the power of the imagination.  
Shelley's life was cut short, and he died in a boating accident at a young age. This fact lends his works, which often explore mortality, a certain weight.