Post-Colonialism means 'after colonialism'. Post-colonial literature is literature written after the decolonisation of countries that were formally colonies bigger empires (other countries, such as France, and the United Kingdom). Algeria was a colony of France, India was a colony Britain. Literatures concerning the after-effects of colonialism on these countries are post-colonial literatures.
The Island is a South-African play. South-Africa is an ex British colony. You need to contextualise your essay according to this.
I am not familliar with this play so unfrotunately I cannot give you a tailord answer. However the following are key features of Post-Colonial literature:
> Passing mixed race woman passing for white.
> discourse - decolonising language, using native discourse
> decolonisation - decolonising the mind. So your thinking isn't based on white (colonial) ideology.
> Rewriting history.
> Diaspora -for example postcolonial diasporas in 'white countries'.
> Hybridity - a hybrid language/hybrid identity.
> Mimicry - the idea of the mimic man should mainly be used when talking about Indian literature.
> Orientalism - orient identities are key in forming western identities. The Occident can only be superior if the Orient is made inferior.