You will be asked for an Apostille when your public documents need to be handed to a foreign country. In this case, you will be asked for a translation as well as the authentication of the seals and signatures of officials on your documents. The authentication is done with the application of the Apostille on the document in question. In this way, the document will be recognized in all the foreign countries that are members of the Hague Convention Treaty.
It is the responsibility of the requesting institution to inform the person whether an Apostille is needed and to provide a clear list of what documents are requested for that specific purpose. However, you do not need to know all the bureaucratic procedures behind it, as long as you make sure that you have the correct documents then your only responsibility will be to find someone who can deal with the Apostille and translation side of things.