Well, you know how when it's your birthday, people usually give you presents and wish you a happy birthday? A birthday card is like a small present that people give you to help celebrate your birthday. It's usually made of paper and has pretty pictures or designs on it, and sometimes it even makes noise or plays music!
Inside the card, people will write a message to you. This message is usually something nice and friendly, like "Happy Birthday!" or "Wishing you the best on your special day!" Some people might even write a funny joke or draw a picture to make you laugh.
When you get a birthday card, it's important to read the message inside and appreciate the time and effort that someone put into getting you a card. And don't forget to say thank you to the person who gave it to you, because it's always nice to show gratitude and kindness to others.