"To extract the juice" basically means to remove or squeeze out the liquid from something, usually fruits or vegetables, in order to use it separately.
Sentences with «to extract the juice»
I mix it up and leave it on the counter for a few hours — I think honey extracts the juice from the onion. (theselightfootsteps.com)
The blades are fairly sharp and specially designed to extract juice quickly. (dietselfhelp.com)
It's awfully hard to extract juice from bananas, even though they're roughly 80 % water. (fitnessdoctrine.com)