A good conclusion should do more than just summarize the main points discussed in an essay; instead, an effective conclusion shows how the key arguments discussed throughout the essay fit together and support the main thesis. Use the conclusion to propose how your essay suggests a solution to the issue discussed, or perhaps how it opens up questions for further inquiry.
Consider someone who, after reading your arguments and discussion, may simply respond with "So what?" To answer this question, make sure that your conclusion makes it clear why your argument matters, and specify why your readers should care about the ideas and arguments you discussed.
Try to make your conclusion one that inspires readers to learn more about things covered in your discussion. You could do so by setting your discussion in a larger context. For example, if your essay is about a historical issue, use the conclusion to compare it to similar issues in the present day.
A good way to make your reader understand that you're concluding your essay is by using a transitional sentence. However, avoid using tired phrases such as "In conclusion..." or "To summarize..." to actively state that you're ending the piece. Rather, return the reader's attention to the arguments you covered. Using a transition is not necessary if you believe it is already clear enough that the essay is coming to an end.