Hot Coolant is a type of fluid that is made in nuclear reactors in heat mode when they are active.
See: Crafting Guide
Hot coolant can be created by venting the heat from a nuclear reactor containing coolant in heat mode. Hot coolant can then be ejected using the Reactor Fluid Port and piped into a Liquid Heat Exchanger together with a Stirling Generator to generate EU. The hot coolant will then be cooled and will become a regular coolant which can be used back in the reactor.
Alternatively, the hot coolant can be used by a steam boiler to make either steam or superheated steam, which can be processed by a steam generator, for a better heat to EU ratio.
Standing in hot coolant is inadvisable as it will burn you, just like lava.