What does SIM locked mean on iPhone?

See this Apple article: How to unlock your iPhone for use with a different carrier

How to unlock your iPhone for use with a different carrier - Apple Support

• Apple can't unlock your iPhone for use with a different carrier. Only your carrier can unlock your iPhone.

You need to contact your Carrier.

Or do you need the PUK code?, See this Apple article: Use a SIM PIN for your iPhone or iPad

Use a SIM PIN for your iPhone or iPad - Apple Support

It means that has a PIN to unlock it. If you didn't set a PIN, some Sim come with a PIN set by the carrier, so get in touch with your carrier and ask

Use a SIM PIN for your iPhone or iPad – Apple Support