A figure of speech which contains an exaggeration for emphasis is called-

Hyperbole is an exaggerated statement or an extreme overstatement.
Example of hyperbole:
“Ten thousand saw I at a glance”
I’ll love you, dear, I’ll love you/Till China and Africa meet,/And the river jumps over the mountain/And the salmon sing in the street- W.H. Auden
It used to create strong emotional responses.
Source: An ABC of English Literature - Dr M Mofizar Rahman