A South Pole is the southernmost geographical point on the surface of a planet. It is located on the opposite side of the planet as the North Pole.
On Earth, it was the location of the continent Antarctica. 20th century-era maps of the South Pole region were stored in the USS Enterprise library computer in 2254. (TOS: "The Cage")
During the 19th century, hundreds of convicted criminals from Ux-Mal were imprisoned on the South Pole region of a moon of Mab-Bu VI. (TNG: "Power Play")
Dialogue from the episode also seems to suggest that the South Pole region of a planet might be a particularly practical place to start a systematic survey.
By 2154, the South Pole of Mars was the target of frequent comet impacts, directed there as part of the terraforming process. (ENT: "Terra Prime")
The Martian South Pole was never explicitly mentioned, but this info can be deduced from the usage of the word "poles" in plural.