650 V MDmesh N-channel — primary-side switch for offline supplies
1.35 Ohm Rds(on) — conduction loss floor at 2 A
Rds(on) is specified at 1.35 Ohm maximum with Vgs = 10 V and Id = 2 A. That is the figure to use for the worst-case conduction loss calculation at the target operating current. At 4 A the on-resistance will be higher — the datasheet curve shows the typical Rds(on) versus drain current — so budget accordingly if you are running near the continuous rating. If your controller outputs 12 V, the ±25 V Vgs max gives plenty of margin.
9.8 nC gate charge — light load on the driver
That is low enough that many PWM controllers can drive it directly through a series resistor without a separate gate driver IC.