From the Preface:
It is a time 59 years in the future. While time travel has not yet been successful, or rather, while time travel has not yet been sanctioned by the Elites, translocation is allowed.
Translocation, or the travel to another location by translocation devices, allows a translocater to move quickly from one place to another at the blink of an eye, between sender and receipient nodes. The technology was not new, but it took time for the Elites to consider and approve it. There are always pros and cons to technology, those who used it for good were able to maximise the outcome for themselves and others in a fair and transparent manner to benefit society as a whole; but there were also those who used it for selfish reasons and benefitted themselves only, or worse, consequently harmed others. Once the laws were passed, once all concerns addressed and loopholes closed, translocation was given the green light.
At the first, translocation made things simpler. People travelled faster. Time was saved. Efficiency was increased many-fold. The Elites sold the technology by its uses for the people, including and especially in the areas of medicine and crime prevention. The illustrations that were commonplace demonstrated how lives could be saved, if a newly-removed organ was translocated a great distance to someone who needed it urgently. It also showcased how the Enforcers were able to instantly apprehend a criminal by sending their police to the scene of a crime with all immediacy, sans travel or horrendous traffic conditions.
And then it drew the attention of a separate race of people, the Reis. This was a race created no different from the humans on Earth, except that a minority of this race of people were borne, imbued, with the Gift. The Rei who had the Gift were telepatically connected to each other, as long as any Rei chose to allow such a connection, so they could be known to each other. These allowed enclaves to form, for good or bad. The hackers of a particular enclave found a way to link the Earth's translocation devices to their own, and sent their people to Earth.
The Elites of the humans learned of the Reis by accident, but by then it was impossible to prevent an invasion. It was not a normal invasion, no, there was no invasion of the physical sort. It was simply takeover. The next thing the Elites knew, the next human was indistinguishable from the next Rei. It became impossible to tell the both apart, and so there was nothing to be done, except accept things for what they had become. And so it came to past that Earth was no longer home to one intelligent race only, the humans. It was now shared with the Reis, or the Reis who travelled to Earth.
The humans coined a name for the Reis, the aliens that invaded their Earth, as "Traveller".