It means 'rider'.
A secondary meaning, 'to tame' is also common.
It is applied after the animal being tamed or ridden: - 'Toruk Makto' - rider of the Great Leonopteryx (Na'vi; Makto) - 'Ikran Makto' - rider of the Mountain Banshee (Na'vi; Ikran)
All Na'vi learn to bond with and ride the Mountain Banshee (Na'vi; Ikran) as part of their initiation into adulthood. Thus all Na'vi adults are in fact, 'Ikran Makto'.
Taming the fearsome Great Leonopteryx, or 'Toruk' in the Na'vi tongue, is a much more dangerous proposition, given the animals size, speed and apex predator status. Only six riders have accomplished this feat; all have been regarded as great leaders by the Na'vi People.