In Arabic, Maskh means for something to change form to an uglier one.  In the Quran and Islamic tradition, this term refers to a specific divine punishment which was sent upon the wrongdoers and wrongdoing nations in the past (of course not all wrongdoers, but those who committed certain wrong acts) which can be called metamorphosis.

Those who underwent metamorphosis acquired the forms of apes and monkeys; meaning the animalistic form of their wrong deeds would take over their human form, making them people with animalistic features and forms.

Most commentators of the Quran consider the metamorphosis it speaks of, real metamorphosis, meaning that the individual would actually turn into an animal, while some Sunni commentators believe it was a change in the nature and the insides of such individuals, meaning that the individual would acquire an animalistic nature on the inside, not that there was any change in form.

The reason for some being inflicted with such a punishment is for others to learn a lesson.  Hadiths tell us that those who underwent this punishment because of their wrong acts didn’t survive for more than several days and that the animals we see today, aren't the descendants of the people who underwent metamorphosis, but the offspring of normal animals.