Thermal pad for TFP401APZP IC
Thermal pads are used to help dissipate the heat generated by the IC die during operation. If the chip overheats it can lead to permanent damage of the device. The thermal pad itself gives you a sort of heat sink which helps dissipate the heat, but sometimes that won't be enough. Sometimes it is required that you also create a thermal pad on your PCB (an exposed section of copper) that the thermal pad of your chip can be soldered to, and then your PCB would act as a heat sink itself. It's not uncommon in high-power ICs for there to be a patch of copper directly under the chip (which solders to the thermal pad of the IC), which connects to an even larger pad on the other side of the PCB using via stitching (a large array of small vias connecting the copper pads together). This greatly increases the surface area of the copper and allows more heat to be dissipated more easily.

Depending on how hard you plan to drive your IC, you may not need a thermal pad. However, I would generally recommend using one anyway just to be on the safe side. Follow the manufacturer's recommended footprint for the device to make sure your PCB's thermal pad is large enough.