Postcolonial theory is of course a massive discipline and it is very difficult to summarise it all in one response, but generally speaking it either takes the form of examining books that have emerged from the postcolonial world that explore the ongoing relationship between the colonial force and the nation and often the chaos that has resulted after independence, or a re-examination of classic texts written in the time of colonialism where issues of colonialism that have previously been ignored or sidelined are pushed into the fore.
A perfect example of this is how Jane Eyre has been re-read in the latter half of the 20th century, with massive focus being placed on the character of Mrs. Rochester as representing the human cost of colonialism and acting as a victim both of the rapaciousness of Mr. Rochester but also of England as a whole in its colonial activity. Instead of viewing Mrs. Rochester in the way that she is presented in the text, such postcolonial views therefore see her as a figure that deserves our sympathy and is not to be dismissed or locked up in the attic so easily. Note for example how Rochester paints a picture of her in the following quote:
I was physically influenced by the atmosphere and scene, and my ears were filled with the curses the maniac still shrieked out; wherein she momentarily mingled my name with such a tone of demon-hate, with such language!--no professed harlot ever had a fouler vocabulary than she: though two rooms off, I heard every word--the thin partitions of the West India house opposing but slight obstruction to her wolfish cries.
The language can't get much stronger than this: Rochester accuses her of being a "maniac" and almost a demon. She is nto only a "harlot" but also more like an animal than human. Rochester has effectively dehumanised her in order to justify his actions in marrying her, enriching himself, and then locking her away in the attic. Mrs. Rochester therefore stands as a symbol of the hidden reality of the wealth of so many upper class individuals who have benefited themselves tremendously as a result of the exploitation of others under the name of colonialism. This is one way of examining this text from a postcolonial angle.