The college I want to apply has an optional supplemental essay. What is it supposed to be about?

Look to see if there is a prompt. If it is open ended think about whether you want to write about the school major or about yourself.

The about yourself is most common and realtes to info you can not really put in the essay or is more abstract. Your essay is about your personality with anecdote but the about self is usually about awards you have gotten in a EC.

School is essentially why this school. Talk about what you value and how that school is an ideal school for you (BS if you need to). I’d recomend to include a semi obscure fact. Like I’d like to join this school becuase insert something about this school (such as the motto a specific program that is unique or a characteristic of the school such as they pioneered X. Basically show interest in the school and show that you researched it more than seeing its rank 10 and has X major.)

Then the major sup essay is why do you want to do X. Write about an anecdote such as if you want to go into X write about what made you want to do that. Try to avoid cliches and write about what you think you can bring to that field.

Example:

By majoring in Engineering I can help solve real world problems and help make my community better. I have wanted to be an engineer becuase I am a practical person and in my EC I have gained X skill and X skill will help me in engineering becuase X.

Also avoid cliches whenever possible.

This is a very basic guide but as the comment above said a lot of supplementals have prompts but the prompts are usually one of 3 listed above.

Comment if you need clarification

You should always do any essay that's optional on the application - it's an extra opportunity to share more about who you are. It sounds like the optional essay for your school has no prompt, or just asks you to share anything you feel is important. In that case, take a look at your overall application. Are there activities that you want to elaborate upon that you haven't covered in other essays? What personal qualities do you want to highlight in your application, and which ones could use more emphasis? It really doesn't matter what the topic is, as long as it adds a different dimension to your application, and is engaging and personal.