The conclusion, like the introduction, should be roughly 10% of the total word count. While there are a number of things a good conclusion should do,there are a number of things it should avoid:
rephrasing the introduction: we should know more at the end of the essay than we did at the beginning, so be more detailed and specific in the conclusion
simply re-telling the reader the points that were made in the essay - what do all the points mean when considered together, more broadly?
ending with a long quote, or many citations from other sources - the conclusion should conclude your essay
focussing on a minor point
introducing new ideas
Instead, a good conclusion should:
draw together all the points that were discussed and consider the 'sum' of their meaning, or, their overall significance
bring closure to the main issue discussed
be a logical ending to what has already been discussed
make suggestions for how the topic needs to be further investigated, or speculate on what is likely to happen in the future