I believe it is an idiom. Literally, it means "on charging become" or "charging is on", but I'm looking for a more meaningful translation. I found the phrase written on the back of a photo of my Russian girlfriend when she was younger (around 30). She is sitting on a hotel room bed in the photo next to a vase of flowers.
Зарядка ,first of all,means morning exercises as soon as you get up and there's another meaning charging gadgets.Both give power and strength. The whole phrase means get ready to do morning exercises! The origin is a song which was popular many years ago.
На зарядку,на зарядку, На зарядку по порядку становись!
It's not only for adults, it is used in gym class by gym teachers to school students. Also, it's a common phrase, that they say on the morning TV, when they show exercises program and 'становись' doesn't mean 'line up', because it is singular and can be said to one person. It means 'stand up' or 'get up', so "На зарядку становись" means "Get up for the morning exercises".