What is this barbaric term?
In sim-swapping we find the word "sim". What do you put in a smartphone? Bravo ! A sim card. So it has to do with the phone!
In short, it's getting your sim card hacked. By exploiting a security flaw, the hacker can get your phone number. I reassure you, this hacking is very rare. The victims are mostly celebrities. But it seems to me that it is worth talking about it anyway.
The main consequence of sim-swapping is that the hacker can access all your personal data. He can therefore see your contact list, your sms, your accounts etc...
The only way to protect yourself is to be careful and secure your phone with passwords. It is also important to remain discreet, not to give too much information on social networks because that is where it starts!