Arial Unicode MS is a special kind of font that has more letters and symbols than regular fonts. Imagine if you had a big box of crayons with every color you could think of, but some of the crayons also had special symbols like smiley faces or flowers. That's kind of what Arial Unicode MS is like for your computer.
When you type, your computer uses special codes to turn your keystrokes into the pictures, letters, and symbols you see on the screen. Arial Unicode MS has extra codes that allow it to display more unique characters than other fonts. This can be especially useful if you are typing in a language other than English that has its own special symbols, like Chinese or Arabic.
But just like not all crayons are the same, not all computers have Arial Unicode MS. You might have to download it or ask someone really good with computers to help you install it. Once it's installed, you'll be able to use all the extra letters and symbols in your work!