At the end of his "Defence of Poetry," Shelley states the following. A few of the words may be difficult, so we will need to unpack this to understand what the last line means:
Poets are the hierophants of an unapprehended inspiration; the mirrors of the gigantic shadows which futurity casts upon the present; the words which express what they understand not; the trumpets which sing to battle, and feel not what they inspire; the influence which is moved not, but moves. Poets are the unacknowledged legislators of the world.
First, hierophants are priests, usually associated with ancient Greece, who interpret mysteries. Unapprehended means not understood. Finally, legislators, a word we might slide over without thinking about, are, on the surface lawmakers, but the important point is that in making laws they bring incarnation or reality to ideas. A new law—such as civil rights legislation—can change reality.
Shelley is therefore saying, first, that poets are priests like those in ancient Greece who interpret what is mysterious or obscure. Second, they are like mirror, reflecting the future—they have a special insight into what the future might look like. They don't, however, always understand what they are saying when they say it: this means they are conduits or transmitters for ideas. If Shelley were born later in time, he might say poets are like radio transmitters. They move ideas to reality from abstraction by stating them in ways masses of people can understand. Therefore, though they are unacknowledged—get no credit for what they do—poets create or incarnate change through the words they write.
Shelley is saying that language has power and that the words of poets have the power to effect change beyond what the poet can predict or understand. Because, like lawmakers in a parliament or a congress, poets cause change, they too are legislators. However, they can make laws—produce change—for the entire world, not one nation.