A SIM card, also known as a subscriber identity module, is a smart card that stores data for GSM cellular telephone subscribers. Such data includes user identity, location and phone number, network authorization data, personal security keys, contact lists, and stored text messages. Your SIM card identifies your device when connecting to your cell network, but it also reveals your identity to various services. 

SIM swapping is essentially the process of hackers activating your number onto a SIM card of their possession. The process helps them take over your phone number, so next time someone tries to access your online banking account, the cyber criminals are the ones receiving the verification passcode instead of you. This is usually effective when someone wants to reset your password or already knows your password and wants to go through the 2-step verification process. This is called SIM hijacking but is also known as SIM swapping and SIM hacking.

When you call your wireless carrier over the phone, the operator usually goes through a quick verification process with you. They often ask for your full name, address, phone number, DOB, and passcode or the last four digits of your social. All of this information has leaked at some point in the past so hackers might have purchased the data from the dark web or might have used other social-engineering ways to get the needed details.