"Pack it In, Pack it Out" is a waste management philosophy of environmentalism. The idea is simple: whatever you bring into a natural area - bags, bottles, trash - must be taken out of the area when you leave.
It is not just removing the trash and biodegradable waste from the natural area, but also the proper disposal of those materials when you get home so that they cause no harm to the ecosystem.
This philosophy is closely related to the broader minimal impact code that is practiced by many environmentalists, naturalists, and conservationists. It has become a part of the philosophy of many organizations that have members who enjoy camping, hiking, trekking, fishing, and other activities that involve entering wilderness areas.