The 'context' of a text refers to the time period in which in was written; namely, the background of the composer.
For example, the context of a Shakespeare play is the Elizabethan era (as this is the composer's background) and not the era in which it is set.
The context influences the composer to think in a certain way about the content of their text, and usually relates to the general ideas of that era in respect to the era the composer wishes to recreate.