I'm new to CA, one part of the CA state tax input on TurboTax that has flagged that I'm not familiar with, it's asking for a 'use tax' amount. What does this mean?

A Use Tax is a tax on goods bought out-of-state on which you did not pay any local sales tax. Generally, if the item would have been taxable if purchased from a California retailer, it is subject to use tax.

Note that this is not just California; many states charge Use Tax now.

See https://www.cdtfa.ca.gov/taxes-and-fees/use-tax.htm