Context refers to that which surrounds a thing or person that works to have an effect on that thing or person. So, the words surrounding a particular word can affect its meaning; but context can also affect people: the time, place and situation surrounding writers can affect both what they write about and how they write about it; equally, the context of the audience can affect the way a text is interpreted. For example, an Elizabethan audience's response to a Shakespeare play is going to be very different from a modern audience's response simply because the two audience's contexts are so very different.
Context means that whenever writers choose to write about something, they are not entirely 'free agents' able to choose to write what they wish and how they wish. The time, place and situation in which they live and write will almost certainly affect several aspects of what they choose to write about and how they choose to do so.
As suggested above, it's very important to remember that readers, too, are affected by their context: our own time, place and situation alters to some degree our reception and interpretation of a text in ways that even its author might not easily have been able to predict - especially when a text is read much later than when it was written or in a different political or social climate. Think of the effects of political correctness these days and how different this would have been for a reader from the past.
Click here to read more about ideology for a deeper explanation of context, author's motivations and themes.