"Im" means "not". So, here it means 'not personal'.
Explanation: "Im" prefix is used to deny or nullify the meaning of the former word and here, as per the definition "im" is denying the meaning of the word 'personal'. To understand the meaning we can refer it to Eliot's "Impersonal theory of poetry" in "Tradition and the Individual Talent" where he is approaching a thought to the poet which is negating his(poet's) presence from the work. He says that the poet should act as a catalyst while writing a work and his/her work should not contain any personal traits of him/her or "Impersonal".
