The first thing that you need to know in order to understand this is that the word "farrow" even in modern English means "to give birth" when applied to pigs. You don't say a sow "gave birth" -- you say it "farrowed."
In this scene, the witches are going to conjure up the apparitions that are going to make predictions for Macbeth. The first apparition is conjured by grease from a murderer and by the blood of the sow that you mention. In other words, we are being told that you conjure these apparitions by throwing into the fire these evil things -- the grease from a murderer and the blood from a sow that has eaten her own babies. This shows us how evil the magic that is going on here is.
Here's how eNotes' "translation" of the lines goes:
Pour in the blood of a female pig that has eaten
Her nine piglets, and throw it into the flame
With fat that has dripped
From a murderer's gallows.
What I would like to add to the previous answer is that the line referred to in your question alludes to Lady Macbeth in a curious way. A female pig that has devoured her nine babies must be a very unnaturally cruel mother reminding us of Lady Macbeth's words in act 1 scene 7:
I have given suck, and know
How tender 'tis to love the babe that milks me:
I would, while it was smiling in my face,
Have pluck'd my nipple from his boneless gums'
And dash'd the brains out, had I so sworn as you
Have done to this.
The strained violence of Lady Macbeth's language here is intended to show her fiendishness as a mother. The mother-pig in the reference cited by you may be taken as an animalish version of that fiendish mother. The grease collected from the hangman's rope which is the other component required to invoke the apparitions is an underhand reference to Macbeth while the sow's blood refers to his accomplice in crime.