Tg is the glass transition temperature of the PCB material and very important parameter.
At this temperature the raw PCB material changes from stiff glass-like material to a elastic and bendable plastic-like material.
The biggest problems in the PCBs during the assembly is the Z-axes thermal expansion as above Tg temperature PCB expands rapidly and if kept long time about Tg temperature the small PCB vias may/will crack internally.
The most evil part is that when the PCB cool off the vias will shrink and may still give unreliable contact like touching together two copper wires, but later with temperature changes, vibration moisture these internal cracks in the vias can cause lost of electrical connections on random basis.
Tg 135 was commonly used and good enough for the old SnPb technology.
Since 2006 the new Lead Free technology require higher soldering temperatures and Tg 135 is not good choice.
PCBs assembled by ROHS lead free technology should be done with material with Tg 170 to be reliable. Especially this is valid for multilayer PCBs with small vias less 0.4 mm.