SIM or Subscriber Identity Module card is is a portable memory chip used in some models of cellular telephones. Like if you get a plan with T-mobile, you need a sim card. It stores your information and phone numbers. and you can switch phones with it. I dont know how to explain it honestly lol. But basically if you get T-mobile phone service you need a sim card. and if something happened to your phone like if it breaks or stops working, you can take the sim card out and put it in your new phone and all your contacts will be there.