Be Careful
Take Care
expression
You take your 気 (energy) and 付ける (attach) it to whatever you're doing, allowing you to focus in and be careful. Since this word is in the te-form, it's a soft imperative. You're basically telling someone, "hey, take care, okay?"
気を付けて is an everyday expression used a lot when telling someone to watch out, stay safe, or be mindful of something. Note that 気をつけて without the kanji 付 is also common, especially in casual writing.
The readings come from the two words 気 and 付ける. So long as you know those from the previous levels, you should be able to read this one as well.
気を付けてね。
Be careful.
気を付けてください。
Please be careful.
気を付けて〜する
to do 〜 carefully
ストーブの火に気を付けて下さい。
Please be careful of the fire in the heater.
今日は雨かもしれないから、外出する時は気を付けてレインコートをもってね。
It might rain today, so be careful and take a raincoat when you go out.
この国の大きな町では、スリが多いから気を付けてね。
Be careful in big cities in this country, because there are many pickpockets.
辺りは明るくないので、足元に気を付けて下さい。
There's not much light around here, so watch your feet.