The Ozone Layer is a layer of atmosphere on many worlds which have a higher concentration than usual of ozone - which is a triatomic molecule comprised of three oxygen atoms. On worlds capable of supporting life, the ozone layer is of vital importance as it absorbs harmful ultraviolet radiation from the sun.
On Earth the ozone layer was located from 15km to 35km above the surface of the planet. Industrialization on Earth resulted in severe damage to the ozone layer in the 20th century, and by the mid 21st century it had been nearly destroyed. The depletion of the ozone layer caused a number of diseases amongst the populace on Earth, including sterility. Earth's ozone layer was eventually restored. (TNG episode: "When the Bough Breaks")
In 2293 over-mining and insufficient safety precautions led to the destruction of the Klingon Homeworld's moon Praxis in a massive explosion. The shockwave from the explosion depleted the ozone layer of Qo'noS. This and other devastation to the planet was a major factor in the Empire's decision to negotiate peace with the Federation later that year. After the Khitomer Conference, work was begun to repair the damage to Qo'noS. (TOS movie: Star Trek VI: The Undiscovered Country; ST novel: Sarek)
The planet Aldea had also suffered the loss of its ozone layer. By the mid-24th century the Aldeans were sterile, and the last child born on that world was a young adult. The Aldeans kidnapped several children from the USS Enterprise (NCC-1701-D), hoping that the children would repopulate the planet. Doctor Beverly Crusher discovered the Aldeans were dying due to the depletion of the ozone layer, and that the Enterprise children would be affected themselves eventually. The Aldeans returned the children to the Enterprise and allowed the Enterprise crew to assist them. The planet's ozone layer was restored, and the Aldeans responded well to treatment. (TNG episode: "When the Bough Breaks")