People going mad about this. Have read the paper it comes from. Highly sensationalized.
What they've found is a certain type of signal which they've interpreted as liquid water.
The technique is to throw radio waves at the planet and process the echo. In 2007, a different satellite found the exact same thing in the exact same place but as it's so cold there, they decided against claiming liquid water.
Since then, there's been some research that shows that certain types of salt are more prevalent on Mars.than was first thought.
So they've took this fact, observed the spot again and declared that it now makes sense that it's liquid water. But it's still a guess and open to other interpretations. It could be quite a few different things that just happen to look like certain under ice lakes in Antarctica look like, which is the reference point they are using.
A Chinese satellite is passing over the South Pole soon so that will probably be used to take more measurements. But to be honest it will probably take a few years to confirm or deny this.