You cannot camp anywhere that you want to along the Appalachian Trail, but there are designated camping sites that you can choose from. There are also around 260 shelters available to stay in across the trail. The majority of shelters will only be available on a first-come first-served basis, which is something to keep in mind.
The designated campsites across the Appalachian Trail are typically in flat areas where you can pitch your own tent or portable shelter. There is also the option for dispersed camping, which is when campers have the freedom to choose the location of their campsite. This isn’t allowed in every part of the trail, but some areas will offer this feature.
There are some people that prefer the solitude and other advantages of dispersed camping, but you will need to check which areas allow you to do so. This is prohibited in many areas, so you should always check to find out what the rules and regulations are.