In Act 1 of "Macbeth," what does "Fair is foul and foul is fair," mean, who is the speaker and what is the context?

This is from Act 1, sc. 1 and the line is spoken by the Weird Sisters (witches).  They say it as a chant and the words set the scene for the play.  The words indicate that appearances will be deceiving - what one sees as "fair" may actually be "foul" and what one sees as "foul" may actually be "fair".  The witches tell us also, in this scene, that they are going to meet with Macbeth which lets us know that Macbeth is going to be the one they mess with.  The words also indicate one of the major themes of the play which is that one should not trust appearances.  There are many references to this idea throughout the play.  Even Duncan makes reference to it in Act 1, sc. 4, when he says, "There's no art / To find the mind's construction on the face." In Act 1, sc. 5, Lady Macbeth tells her husband to put on a false face because his face tends to show his emotions - "To beguile the time, / Look like the time;...look like the innocent flower, / But be the serpent under't."  Macbeth's final words in the first act are, "False face must hide what the false heart doth know."  Before he and Malcolm flee Scotland, Donalbain says, "There's daggers in men's smiles;". Of course one of the major examples of this theme is in the visions that the weird sisters show Macbeth in Act 4, sc. 1.  Those visions give Macbeth a false sense of security which lead to his death at Macduff's hands.