In the discussion of the question "Nuclear Fusion with extremely high pressure and low temperature", the conclusion is only ultrahigh pressure cannot trigger fusion. But why does NIF try to use 192 laser beams to trigger capsule ignition? There is no source to heat fusion fuel.
The lasers provide the heat, as the laser energy is absorbed by the outer surface of the fuel pellet. The surface of the pellet is heated enough that it becomes a plasma, and as it expands, it compresses and heats the rest of the pellet. This is aided by a propagating spherically-symmetric shockwave that propagates inward after the initial laser impact; the shockwave, when reaching the center, compresses and heats the center of the pellet even more, ideally to fusion temperatures.