Basically, it means that the army has settled in for the winter in a semi-permanent camp. With certain notable exceptions (the Fredericksburg and Fort Henry & Donelson campaigns, Stones River), Civil War armies preferred not to campaign in the dead of winter. The roads were often bad, forage was invariably scarce, and the weather took a heavy toll on the men. Sleeping on the ground with nothing more than a blanket and - possibly - a greatcoat when it's 20 degrees outside is no fun at all and, what's more, bad for the health of the army. Sick, frostbitten soldiers are generally something a commander wants to avoid unless he has a very good reason to risk it. So the army would halt, construct temporary shelters, often taking the form of crude log cabins, and await the spring campaigning season.