Paradise Lost-Book 1 by Milton Line (1-26) are the prologue, there are three more prologues in Paradise Lost - (Books III, VII and IX). Milton follows the classical example of Epical poetry and invokes a Muse.
A Muse was invoked by a classical poet to help him in the task of writing his poem. Accordingly, Milton asks his Muse to lead him higher than the Aonian mount of the classical poets, because the subject of his epic is higher than theirs. The prologues in the Paradise Lost begin as classical invocations but, with one exception, they rise to Christian prayers to the Holy Spirit.
In first twenty-six lines, we are acquainted with the theme of Paradise Lost, man's first disobedience; we learn that the materials are to be taken chiefly from Genesis, that Milton is writing a classical epic but that he intends, with the aid of the Heavenly Muse, to transcend the classical, and in a poem both Hebrew and Christian, deal with the most profound of all problems, "to justify the ways of God to men". In twenty-six lines, Milton has fused three great civilizations, the main sources of Renaissance religious poetry: classical, Hebrew and Christian.
Reference: Paradise Lost Book I critical study by Ramjilal.
Lines 1-26 in Book I are just the invocation of the Muses (a characteristic of epic poetry). Milton calls on the spirit of God to aid him in the relating of this theodicy (defn. a justification of the ways of God to men). He says that he wants to write an epic greater than any before, and (arguably) he does.
See the invocation in Homer's The Illiad and The Odyssey and Vergil's The Aeneid for other examples of the invocation.