Welcome to CalPERS.
Pre-tax means that your retirement contributions are deducted from your pay before your federal and state income taxes are assessed. Pre-tax is another way of saying, this will reduce your taxable income.
So, if your gross pay were $4,000 per month:
$4,000-$373 = $3,627. You would then pay income taxes on the $3,627.
If you're a new employee, this is an EXCELLENT time to enroll in the 403(b), and reduce your taxable income further, while growing your 'stache!