"OTG" stands for "On-The-Go" and it is a specification for USB (Universal Serial Bus) devices that allows them to act as either a host or a peripheral device.
In a typical USB connection, one device acts as the host and the other device acts as the peripheral device. The host device initiates the communication and provides power to the peripheral device. The peripheral device, on the other hand, responds to the communication and typically receives power from the host device.
With the OTG specification, a USB device can switch between acting as a host or a peripheral device, depending on its current needs. For example, a smartphone with OTG support can act as a host when connecting to a USB flash drive to read or write data, and then act as a peripheral device when connected to a computer to transfer files or charge its battery.
To support OTG functionality, a USB device must have an OTG port that allows it to switch between host and peripheral modes. OTG cables are also available that allow devices with non-OTG ports to connect to OTG devices.