Def- Speaker is the voice or point of view from the poem.
Ex. In the poem "If" by rudyard kipling
the speaker is the father talking to his son
If you can fill the unforgiving minute
With sixty seconds' worth of distance run -
Yours is the Earth and everything that's in it,
And - which is more - you'll be a Man my son!
The speaker is significant because it tell the point of view of the poem.