Net filtration pressure is the force driving fluid out of the capillary and into the tissue spaces; therefore, it is equal to the difference between which two pressures?

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Net filtration pressure reflects the balance of forces across the capillary wall: hydrostatic pressure in the capillaries pushes fluid out, while the osmotic (colloid) pressure from blood proteins pulls fluid back in. The dominant outward force is capillary hydrostatic pressure, and the main inward force is blood colloid osmotic pressure. Therefore the net outward driving force is the difference between these two: capillary hydrostatic pressure minus blood colloid osmotic pressure. In the full Starling framework you’d also account for interstitial pressures, giving a more complete expression, but the simplified form used here compares those two pressures.