What the 40 mΩ Rds(on) means for your switching design
At 16 A the Rds(on) is guaranteed to stay under that ceiling; the 34 nC typical gate charge at 10 V tells you the gate-driver needs to push about 34 nC per switching cycle, which is moderate for a D2PAK device and keeps the transition losses manageable in a 50–100 kHz converter.
D2PAK footprint and thermal reality
The part comes in a D2PAK (TO-263) surface-mount package with the drain tab as the exposed pad. The 3.8 W at ambient (Ta) number is for a free-air board with minimal copper — a realistic derating guide if the layout is cramped.
Obsolete — the sourcing picture
International Rectifier lists the IRFZ34NS as Obsolete. If a pin-compatible replacement is acceptable, the IRFZ44N is a common cross-shop candidate but its package (TO-220 through-hole) differs, so the board would need a layout change. No surface-mount equivalent is officially documented.