Let's break it down:
* Em: This is a contraction of "en" meaning "in" or "on".
* mi: This means "my".
* mochila: This means "backpack".
* tengo: This means "I have".
* unos: This means "some" or "a few".
* bolígrafos: This means "pens".
So the entire phrase translates to "In my backpack I have some pens".