Context is the conditions, situations, and circumstances that surround an object, an event, or a segment of language. In other words, context is similar to an environment.
For example, consider a sentence as the context or the environment of a statement of language. If you are uncertain of the definition of one of the words in that sentence, you can use the context of that sentence to determine what that word means. The best way to show this is using a word that has multiple meanings.
"He had a coughing spell."
"The witch cast a spell on him."
Since 'spell' has multiple meanings, you have to use the context of the sentence to determine the meaning. You can look up the definition in a dictionary but you inevitably have to use the context of the sentence to get the particular meaning.
Context is also used more broadly. You might want to know what civil rights meant within the historical and cultural context of the 1960s in America. Obviously, civil rights will mean something different during this time as opposed to what it meant in 1800 when slavery was still legal.
You can read things in different contexts. Some critics will talk of this as reading things with different "theoretical lenses" which is simply a more academic way of saying "to read in a different context." For example, reading a poem such as "The Red Wheelbarrow" in a Marxist context might suggest that the poem makes a statement that too much of life depends upon the accumulation of capital.
so much depends
a red wheel
glazed with rain
beside the white
In other words, too much depends upon things (wheelbarrow) because those things are used for production, and to make money. Read in a Romantic context, the suggested meaning might be that so much of life depends upon the way human life and nature interact. In any reading, the meaning of any idea, object, event, or any portion of language (from a word to a novel) largely depends upon the context.
Keep in mind that word in a sentence comes with a context. The sentence is the context. But you can also add a second context (theoretical lens) to that sentence which might change the meaning more. For example, consider the statement "The story of his attempt and eventual failure was epic" will be interpreted differently in different contexts. In a modern context, in popular culture, this could mean anything from losing a bid to the presidency to trying to dunk a basketball and failing; this is because popular culture, "epic" is currently being used to mean anything even slightly dramatic. But that same statement considered in the context of ancient Rome will mean something very different, with connotations of heroes and warriors.
Context is used differently in different subjects, but in English it concerns to main topic and where in all related things or its conditions are described just to add more information in nut shell to the readers for knowledge sake. It is mostly important for the readers having a little knowledge about the selected topic.