What does "First Language" mean?

Applying to College Common and Coalition Application
1

<p>Does it mean american born’s first language should be english?
I speak two languages. It feels like they are all my first language.</p>

2

<p>An American born’s first language is not always English. </p>

<p>Choose English if that’s one of your two languages. Otherwise, you might fall into the TOEFL mishap.</p>