Originally, a subaltern meant someone who is lower in rank, like a colonel to a captain. However, it became associated with someone from a lower socio-economic class. 

In Postcolonial Theory, from the work of Gayatri Spivak and her essay "Can the Subaltern Speak?" she looks at the application of the word within the context of colonialism. When the colonizer speaks for the colonized, they become the subaltern. As a result of colonization natives of colonized lands became subalterns in their own homes with the arrival of Europeans.

In answer to the question of her essay, Spivak essentially argues that no, they cannot speak, for the colonizer speaks for the indigenous person.