For children under age 18 and certain older children, unearned income over $2,100 is taxed at the parent's rate if the parent's rate is higher than the child's. If the child's unearned income is more than $2,100, Form 8615 is used to figure the child's tax.
Apparently you have sufficient unearned income (interest, dividends, etc) to require you to complete this form for 2016.