Would you explain the following statement? Poetry has been defined as “the expression of elevated thought in elevated language.”

'Thought' refers to the 'content'/ 'matter', and 'language' refers to the 'form' / 'manner'. The definition you have cited significantly contains the word 'elevated' as a common qualifier for both 'thought' and 'language' in poetry.

Something 'elevated' means that it is higher than the ordinary or commonplace, something lofty/noble/rarefied/transcending. If poetry is 'elevated thought', it must contain ideas which aim to transcend the limits of our mundane life, our lived existence. In that sense, poetry liberates us from the petty trivialities of life; it sublimates; it charts out a journey beyond, into the world of imagination, beauty, truth & joy.If poetry is an expression in 'elevated language', it means that the language of poetry must be somewhat vertically distant, essentially paradigmatic, to use the phrasing of Mallarme, 'to purify the dialect of the tribe'.

This definition of poetry is characteristically classical, may be very true of Milton's poetry, reminiscent of Longinus's notion of the 'Sublime'.