"Call me when I'm down."
Here's a breakdown:
* bom: This is a colloquial way of saying "good," but in this context, it's more like "okay."
* quando eu estiver para baixo: This translates literally to "when I am down," meaning "when I'm feeling sad or depressed."
* te ligo: This means "I'll call you."
So, the full phrase expresses the idea of someone offering support to another person by saying they will call them when they are feeling low.