Tquarks are tangled quarks, which are used for faster than light communication. A pair of tangled quarks (or entangled, as it's more commonly known) can be separated, but remain linked in some way.

When a hauler carries a cargo of tquarks, she takes one half of a pair, while the other half remains behind. In this way, a pair of tquarks make a connection between one planet and another.

A 75-ton container of tquarks actually only contains a few grams of the substance; the rest is taken up by material necessary for transporting the tquarks safely without them becoming un-entangled. But there are still gazillions of tquarks in that few grams (the exact number is far too long to write here, but it is something to the order of 10 to the power 23) so over the years a vast number of connections have been formed between different places in the Solar System,. These now take the form of a tightly-woven network which allows messages to be sent all over the Galaxy without having to worry about the speed of light.

Hence the instantaneous transmission of messages using comm units - without which, Armstrong Cuthbert would not be able to maintain their business.