The word "blithe" is an Old English word literally meaning 'carefree, happy and lighthearted.'
"Spirit" of course would mean 'an incorporeal supernatural being.'
Shelley begins his poem by saluting and greeting the skylark by calling it a "blithe spirit," because the skylark is a bird which is rarely visible and only its melodious song is heard by the people. The sweet song of the skylark reveals to Shelley that unlike ordinary mortals like himself it's absolutely carefree.
The skylark symbolizes the free soul .It , according to Shelly's vision is ,ignorant of the sufferings and cares of this world .Blithe means ignorance of worldly desires , and that of the hopes and fears of the people of this world ."Spirit", suggests ,-soul .Hence the phrase , "Blithe spirit",simply speaks of the disembodied soul .
in the poem " to a sky lark " Shelley is listening to the song of a bird,which is itself invisible. it seems to the poet that the bird, while singing, soaring high above the ground,has it lost its physical existence and has become a spirit.Shelley is here trying to represent the bird as an abstract quality of pure joy, a quality so poignantly missing in the humans