Much of Earth's freshwater is stored as ice, mostly at the poles. The North Pole is covered by floating pack ice, while the South Pole is covered by the Antarctic ice
sheet. This ice sheet, which covers an area of nearly 14 million square kilometers,
holds about 90% of the freshwater on Earth's surface. During warm months, some
of the polar ice melts, yet global sea levels stay roughly the same. Why does this
occur?

The ice caps are already in water.

Explanation:

The ice packs in North Pole and the ice sheet in Antarctica are already in water. When they melt therefore, they are simply being converted from one state to another but the quantity is the same.

In other words, the volume that the ice occupied in the water becomes the volume of liquid added to the water so there is no net change in volume added leading to sea levels staying the same.