Eruvin Made Easy part 1: Does Your City Have Doors?
The biblical prohibition of carrying is carrying between a reshut harabim and a reshut hayachid. Included (possibly a halacha l'Moshe MiSinai) is carrying 4 amot in a reshut harabim.
MiD'rabbanan one may not carry from one reshut hayachid to an adjacent reshut hayachid on Shabbos (without an Eruv Chatzerot). Our eruvin are made using doorways. A reshut harabim can't be made into a reshut hayachid except with actual doors that can be locked at night.