There is not a whole lot of details on exactly what the Traveler could do, or how he could do it. It's also never explained what, specifically, he did in the finale, other than "fix the timeline." However, by piecing together all his mentions/appearances in the past two seasons, a somewhat more complete backstory emerges.
The Traveler was from a future that is implied to be
the same one that Kiera eventually ends up in.
This is not the future that Kiera initially came from, however. That future -- the one we see at the start of the show -- was created when the Traveler did "something unspecified" while time traveling, and broke his own timeline so that his future no longer existed.
The Traveler apparently has extremely advanced time travel technology embedded directly into his body, and also has some form of psychic abilities (technological or natural isn't clear.) For example, during the show we see him:
- Travel arbitrarily through time and space; e.g. he randomly appears wherever he happens to need to be.
- Mentally speak to people (such as the leader of the Freelancers), and even reach across time streams
such as when put young Alex and old Alex in mental communication with each other.
- He seems to have some level of precognitive ability, and "knew" when certain things happened that would help him achieve his goal, such as the arrivale of Cameron and Liber8, or
the arrival of the the future soldiers from the new timeline.
- He knew exactly how to use advanced technology in ways that I don't think other people would have understood; in particular,
he used the time ball to fix the timeline and get himself home (it's not immediately clear, but this is what he has in his hands in his brief appearance in the finale)
Once he realized what had happened to his timeline, he traveled further back in time and began recruiting the Freelancers, guiding them to try and repair the timeline and prevent any more damage to it. When he saw Liber8 and Cameron arrive in the past, he realized that this was his chance to completely repair the timeline back to normal. By using his Freelancers to force Kiera and Liber8's hand, he was able to guide the "present" in such a way that led him to achieve his goal. This meant that he had to:
- Make sure that Liber8 was not successful in producing their future
- Ensure that Alex Sadler's future self still invented time travel
(by sending young Alex forward in time to give him the idea)
- Ensure that Kellog took over SadTech and eventually
sent the future soldiers back in time with their time travel technology.
Once all the pieces were in place, he was able to do "something" with the time travel portal such that he
and Kiera ended up in the future the Traveler was originally from, and those in the present could work towards creating it.