This is called tanwin or nunation. Long story short, it is a type of vowels in Arabic (and I don't know why Duolingo is not putting the marks here).
Tanwin can come in 3 flavors: -un, -an and -in depending on the status of the word (nominative, accusative, ...etc) when the word is indefinite. Thus, you can somehow think of it as being a marker for the indefinite noun (like a/an in English somehow) - though this is just a short explanation.
The sign for -un Tanwin is:
or maybe some books (specially for children) would write it as:
Thus, the vowel you've heard before smart is actually the end of the adjective before it (amríkiyatun) - American (f.adj.).