In the play "Macbeth", what do the ingredients in the cauldron in Act 4, Scene 1 represent?

Great question. There are three main properties to the ingredients:

The first witch puts in ingredients which are hallucinogenic, creating (presumably) the apparitions which appear to Macbeth. 

The second witch adds animal parts dismembered from animals, and with a particular emphasis on the disgusting (eye of newt, toe of frog).

The third witch initially adds parts of animals which are a little weirder than the second witch's (scale of dragon!) but then adds in human body parts all of which have an anti-Christian theme: the Jew,the Turk and the Tartar are all non-christians.

So, in short, the ingredients represent things which bring on hallucinations, disgusting parts of supposedly poisonous, or magical, animals, and finally, things which specifically make the witches' brew a heathen, unChristian potion.

The website below has fantastic information about all of the individual ingredients that the witches put in. I recommend it.

Hope it helps!