A CharlieCard is a special card that you can use to pay for your trips on the subway, buses, and trains in Boston, Massachusetts. It looks like a small plastic card with a picture of a train on it.
When you want to ride the subway, you just put the card on a special machine next to the turnstile. The machine reads the card and deducts money from it so you can go through the turnstile and catch your train.
The same thing goes for buses and trains too! You just tap your card on the payment reader and it will deduct the fare for your trip.
You can also reload your CharlieCard with more money at stores or online so that you can keep using it for future trips.
In conclusion, a CharlieCard is a little card that helps you pay for your rides on public transportation in Boston, so you don't need to carry coins or paper money around with you!