A TF card on an Android tablet is a SanDisk-specific brand of microSD card that provides additional storage space for the device. As of 2015, the microSD is the smallest flash memory card available. It is also commonly used in other multimedia devices, such as smartphones and non-Android tablets.
SanDisk referred to the microSD cards as TransFlash or T-Flash cards prior to their official product launch. The microSD card was officially introduced in 2005 and, as of 2015, can have storage capacities of up to 512GB in its microSDXC Extended Capacity variant. MicroSD cards can be placed inside an adapter to allow them to be read through normal SD card slots.