The transition of a substance from the gaseous to the liquid state is called condensation. The product of a condensation is called condensate.
This occurs when the temperature inside the pipe is not high enough and the resulting exhaust liquefies. In the case of condensate in the exhaust system, i.e. in wet mode of operation, it is necessary to install a condensate tray with a drain in order to be able to remove the resulting liquid.
In double wall exhaust systems, the risk of condensation is relatively low, since the high temperature in the inner tube is maintained by the insulation in the outer tube (= chimney effect).