There’s a lot of reading in the article this originated, so I have extracted just this one section. I’d call it a ‘tl;dr’, but it’s still very long.
Let’s say you go to Dave’s Cafe for your morning coffee and a little emailing before you head into the office. Dave’s a smart guy, so he’s secured his wifi with a password and encryption. While you’re waiting for your coffee your laptop connects to “Dave’s Place” wifi network and everything is set. Now a hacker has decided to target people who go to Dave’s Cafe so he sets up a wifi access point nearby and also calls it “Dave’s Place” and even uses the same “coffeeislife” password. If the rogue point is close enough to the cafe, people might connect to it by chance, or if the hacker makes his network signal stronger than the real Dave’s Place more people will connect to the Evil Twin because devices pick the strongest signal to connect to when given a choice. Even more clever, the hacker could set up the wifi just out of range of the real Dave’s Place wifi and as people are coming by, their smartphones and other devices will automatically connect to it thinking it’s the real one. Now you’re thinking “What’s the problem? My wifi connection is secure, I had to put in my password just like you said.” The problem is that a wifi password only encrypts the data between you and the router, once the data gets to the router it’s decrypted. Anyone who is directly connected to the router with an ethernet cable, can see, and monitor, all the information going through it on its way to the internet. There is a lot of information to be gleaned that way and because the hacker controls the router—therefore the connection to the internet—he can also redirect you to whatever website he wishes. This is called a “man in the middle” attack.
This is why I don’t connect to open connections in Westfields or malls.
I suggest you read the full article to fully comprehend the situation.