A walls in insurance policy coverage for a condo is basically like regular homeowners insurance you would buy for your home.
The walls in insurance policy covers the interior of your condo and any personal property inside your condo and usually the rest of the condo has a building insurance policy that is purchased and held by the main owners of the condo property.
That would be in case a natural disaster or fire destroyed the entire condo the main condo insurance of the building would cover that.
But if you have walls in condo insurance then your belongings and the interior of the building would be covered in case of damage to the condo.