In act 4 scene 1 of Macbeth, we find the witches concocting a potion, and through these lines we learn that they are tossing some pretty interesting ingredients into their mixture. To understand the full context, we need to analyze another line:

Witches' mummy, maw and gulf
Of the ravined salt-sea shark.

Witches' mummy is the literal mummified flesh of witches. Long before modern medicine began to take root (with an advanced understanding of illness, bacteria, viruses, and germs), people looked to all sorts of things to cure their ailments with remedies steeped in superstition, even to the point of consuming mummified flesh.

"Maw" and "gulf" are older English words that mean stomach and throat. Combined with the next line, these two added ingredients are the stomach and throat of a great shark.

They throw equally grotesque objects into the cauldron of this particular mixture, including a Turk's nose, a baby's finger, and goat's bile. "Double, double toil and trouble," indeed.