Imagine you have a set of cards, each with a picture or a word on it. Now, let's say the metacard is one special card that tells you about all the other cards in the set. It basically gives you important information about how to use all the other cards together.
Similarly, in the field of computer science, a metacard is a program or a set of instructions that tells other programs how to work together. The instructions could tell the other programs which data to use, how to access it, and how to collaborate with each other.
Basically, think of the metacard as a boss who tells all the other programs what to do and how to do it. It helps them communicate and work together more efficiently.