TJ-MAX is the highest allowable temperature the junction in the IC is allowed to achieve. TA is defined as the ambient temperature of the air in the vicinity of the regulator. RTHETA-JA is the thermal resistance from the junction to ambient. This includes the thermal resistance from the junction-to-case, the case-to-PCB, and PCB-to-air. VFB is the feedback voltage used to set the output voltage. R1 and R2 create a resistor divider to form VFB from VOUT. The error amp works to keep VFB equal to VREF.