The scene you're likely thinking of is when the children are preparing to travel through the tesseract. Mrs. Whatsit, Mrs. Who, and Mrs. Which all have unusual ways of perceiving the world and require special tools to navigate the universe.
* Mrs. Whatsit needs her spectacles to see clearly through the tesseract. She mentions that her glasses are "very special, you know... They help me see the way things really are."
* Mrs. Who relies on her books, which contain different verses of the Bible that she can use to access different realities.
* Mrs. Which is the most powerful and can see the universe without any external aids.
Meg, being a normal human, doesn't need any special tools to travel through the tesseract. She gives Mrs. Whatsit her glasses because Mrs. Whatsit needs them to see. This is a small but important act of selflessness and trust that highlights Meg's developing character.