Lest means “for fear that” or “to avoid the risk that.” It's a somewhat formal word people use when warning about something they want to prevent from happening.

You might read in a story: “The hikers checked their map carefully, lest they get lost in the woods.” This means they checked the map because they didn't want to get lost, or to make sure they wouldn't get lost. Another example: “She wrote down the instructions, lest she forget them later.”

The word always points to something you want to avoid. When a teacher says “Take notes during the lecture, lest you miss important information,” they're warning you about what could happen if you don't pay attention.

Lest is more common in writing than everyday speech. You'll find it in older books, formal speeches, or careful writing. Most people today would say “so that we don't” or “to avoid” instead. But when you encounter lest in your reading, you'll know the author is giving a warning or expressing concern about preventing something unwanted from happening.