TFLOPS stands for Tera Floating-Point Operations Per Second. One TFLOP = one trillion (10¹²) floating-point math operations every second.
Think of it as the GPU's horsepower rating. A car's horsepower tells you how much mechanical work the engine can do. TFLOPS tells you how much mathematical work the GPU can do.
A floating-point operation is any basic math on decimal numbers: addition, subtraction, multiplication, or division. When a GPU has 990 TFLOPS at FP16, it can perform 990 trillion of these operations every second at half-precision (16-bit) floating point.