The OTG USB Adapter is used to connect devices which have micro USB with full size USB device. For example, if you have an Android tablet which only has a micro usb port, but you want to connect your full size USB mouse or keyboard to that device, then you need an OTG USB Adapter.
Currently, almost of all the Android tablets only have micro USB port, so the factory can make the tablets as thin as possible. Some windows tablets also only have micro USB port. As I know, only the Microsoft Surface tablet has both micro USB port and full size USB port.
To connect full size USB device to your Android tablets, you need this OTG USB Adapter. A simple adapter only has one micro USB male and one full size USB female. After you connect the micro USB male to your android tablet, you can connect any full size USB device to the other USB female port. However, there could be one problem. As you connect your USB mouse or keyboard to your android device through OTG USB Adapter, you android tablet will provide the power to the USB mouse or keyboard. It could make the tablet battery running out very fast.
There is one solution to connect your full size USB device to your micro USB port tablet and keep your tablet battery full at the same time. You can use this special OTG cable adapter, the Hittime 3 in 1 USB OTG Cable Adapter. This adapter has one micro usb male which connect to your device. It has another two full size USB female port to allow you connect two USB devices to your android tablet. It has another one micro USB female to accept charging.