So how can
Be careful or
Is it something like "Take and keep" that means that the person need to take all information and keep it for it self?
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Garde has not the to keep meaning but the to guard one, so it's more like be on (your) guard (literally "take guard", i.e. take care, pay attention).
Beware that prendre garde often keeps the same meaning regardless of whether it is used negatively or positively.
Garde has several meanings https://www.larousse.fr/dictionnaires/francais/garde/36093
What interests us is the last definition :
- En boxe, en escrime et dans les sports de combat, attitude que l'on prend pour engager le combat et se protéger.
You can understand
Prends garde has
Take a position that allow you to defend yourself.