"Don't forget... (something important)"
Here's a breakdown:
* Lest: This word means "for fear that" or "in case that." It expresses a concern about something happening.
* You: This refers to the person being addressed.
* Forget: This refers to the act of failing to remember something.
Example:
"I'm giving you this list of instructions, lest you forget anything important."
This means the speaker is concerned that the person they are talking to might forget something important, so they are providing a list of instructions to prevent that.
Other similar phrases:
* "Don't forget..."
* "Remember..."
* "Keep in mind..."
These phrases all convey a similar meaning of reminding someone about something important.