A phrase one might hear upon entering a national park or nature preserve of some type, especially if one is going to make an extended camping expedition within said park or preserve. "Pack it in, pack it out" refers to the safe disposal of non-biodegradable waste over the course of your trip. Basically, if you generate any non-biodegradable waste, you have to bring it back from the preserve with you. In a more general sense, you have to bring back all the supplies you took in, whether they are waste or not. This helps to preserve the park by discouraging any sort of human-generated pollution.
This stricture often limits the items that people bring into parks in the first place. Non-biodegradable soap, for instance, is difficult to keep out of the water, even if you do wash as far from shore as possible: you may still infect the groundwater in the area. So you simply bring only biodegradable soap, such as Ivory.
Biodegradable waste (such as food and human waste) can usually be left in parks, as long as it is safely disposed of, that is to say, buried a safe distance from any water. This is fortunate, considering how long you might need to drag your waste around with you; it would get very messy very quickly, and attract animals to boot.
However, you will probably still end up bringing a variety of messy things around with you, especially if you are a girl. Tampons and pads, for instance, must be taken out of nature preserves. So must cigarette butts. Make sure you bring enough plastic bags to contain your waste safely. You should also make sure to store your waste well overnight, to discourage any animals around. Store it like you store your food and you should be fine.