Every year, anyone who earns wages in the US must declare their total earnings to the federal Internal Revenue Service and the state of California and pay any taxes owed - or be refunded any overpayment of taxes (taken out of each paycheck).
If you earned any income from the US during the calendar year (January through December), you must file an Income Tax Return using IRS form 1040NR to the federal government and form 540NR to the state of California. Scholarships/fellowships, summer internships, summer research, study grants, travel grants, gift cards all count as "income," depending on the tax treaty between the US and your country.
Additionally, all international students must file a Form 8843 if you were in the US as an F student for even a short time during the year, even if you didn't earn any wages in the US and don't need to file 1040NR.