"The Tyger describes what a frightening creature the tiger is and questions over and over what kind of god could make such an animal.
In the second stanza, the poet asks what god ("immortal hand") would have dared to seize the hot, burning fire that created the tiger's eye? In stanza three, the poet wonders who could make the fear-inspiring heart of a tiger? In the next stanza, he asks who could make its terrifying brain? The poet uses words like "dread" and "deadly terror" to express how frightening a tiger is.
The fifth stanza is the heart of the poem. Here, the poet asks
Did he [God] smile his work to see?
Did he who made the Lamb make thee [you]?
Blake wonders if God could have been happy ("did he...smile") at making such a bloodthirsty beast, which preys on other animals. Blake tries to understand how a God who could create an animal as innocent as a lamb could also make an animal as fierce as a tiger. It doesn't seem to make sense to him.
The last word of that stanza, "thee," is ambiguous because it could more than one meaning: is the poet questioning the tiger or is he questioning the "thee" (you) reading the poem? If he is questioning the reader, is he suggesting that people are as fierce and predatory as tigers?
Finally, the poem ends by repeating almost exactly the first stanza, questioning what god could make the tyger, "burning bright." But, very significantly, one word changes. In stanza one he asks, who "could frame thy fearful symmetry" but in the last line he asks, who "dare frame thy [your] symmetry?"
This word shift is subtle but shows the poet's willingness to challenge God more directly by the end of the poem. If "could" means capable, as in who is capable, who has the skill, to make a tiger, "dare" is about choosing to do something so dangerous: why God, the poet asks at the end, did you choose to make something so dangerous? Behind the question is a larger challenge for God: why did you choose to create a world so filled with danger? Blake doesn't answer the question.