Context is the positioning of a word or phrase. This can be used by the teacher to help students gain an understanding of its meaning.
For example, look at this sentence.
I saw a bright red fligret in the sky.
This contains the unknown (and non-existent) word, fligret. However, we can look at the context in which it occurs to try and work out the meaning.
This suggests that a fligret is possibly a bird. Or perhaps a type of aircraft.