What is a weak security mean on my wi-fi

A "weak security" message is telling you your router is set to use older network security settings. Note that you are not seeing this because your security has changed on your network, just that the new system software version on your Apple device is now programmed to analyze your wi-fi security and report it to you.

See this support article about 'Recommended settings for Wi-Fi routers and access points'

Click here --> https://support.apple.com/HT202068

"WPA3 Personal is the newest, most secure protocol currently available for Wi-Fi devices. It works with all devices that support Wi-Fi 6 (802.11ax), and some older devices.

WPA2/WPA3 Transitional is a mixed mode that uses WPA3 Personal with devices that support that protocol, while allowing older devices to use WPA2 Personal (AES) instead.

WPA2 Personal (AES) is appropriate when you can't use one of the more secure modes. In that case, also choose AES as the encryption or cipher type, if available."

Change the settings on your router to a minimum of WPA2 Personal (AES) if possible. You may be able to set them higher which is okay as long as you do not have any old other hardware needing to connect to your router.

You configure a router by logging into it with a web browser. Review your router user manual for instructions. If you have not changed the password it may be printed on a label attached to the router. This password is not the same as the one you use when connecting to wi-fi.