Onewire, otherwise known as a MicroLan, is a communication bus which uses only one line for power and transmission of data. Of course, a common ground is required meaning a second wire in the majority of cases.
A Onewire bus consists of one master and at least one slave device. All devices make their connection via an Open Drain connection, with at least one pull up network to a positive supply voltage, of which 5V or 3V3 is typical. Hence the resting (“idle”) state of the bus is at the supply voltage.