This quotation is from Act 1 Scene 7. The full quotation is:
Besides, this Duncan hath borne his faculties so meek, hath been so clear in his great office, that his virtues will plead like angels, trumpet-tongued, against the deep damnation of his taking-off; and pity, like a naked newborn babe, striding the blast, or heaven's cherubim, horsed upon the sightless couriers of the air, shall blow the horrid deed in every eye, that tears shall drown the wind. (I.7.16-25)
A modern English translation might read: Duncan was such an honorable King, so clear and purposeful in his position, that his kingly virtues will loudly speak for him after his death against the wrongs that have been done to him; and pity will be carried among the people on the wings of angels so that everyone will know the details of the king's horrid murder, and the people's tears of sadness will take over everything.
In terms of the plot of the play, this quotation suggests that Macbeth's murder of King Duncan will not go unknown or unpunished. Macbeth will never be an honorable king like Duncan, and eventually all the people will know of his treachery.
This is just to add on to the implications of the quotation and its overall dramatic significance. In these lines, Macbeth is dwelling on the moral criminality of the act of Duncan's murder. He cannot justify the act in any way despite his repeated attempts. In the quotation, the comparison between the abstract quality of pity and the stark image of a naked new born baby is crucial as this pity, aroused in the mind of the masses of the nation would corrode Macbeth's project in the long run. The baby-image echoes Lady Macbeth's rather violent evocation of the baby in the context of her utterance on killing her own child at the time of breast-feeding. Mark the epic canvas and the mythical imagination here. Macbeth imagines a flight of this child in the air riding on the waves of the wind. The influence of the witches is evident in Macbeth's rather spectral imagery. The hyperbolic quality is notable too. The baby metaphor brings out the innocence of Duncan's personality and the strange pity of nemesis it may raise in the people of Scotland.