Index cards are stiff pieces of paper, usually 3 x 5 inches. They can be blank or with lines on them.
For homework, you might:
* write a topic at the top (like "Water: Universal Solvent") and subtopic sunderneath (like "dissolves salt, dissolves sugar" etc.).
* write the title and author of a book at the top, and a quote from the book underneath.
* write the name of something ("hydrogen atom") and draw or glue a picture underneath.
Your assignment should explain what kind of cards to make. If it doesn't, ask your teacher for more detail.