Why does it say “weak security “ when I connect to my WI FI

You are being notified that the WiFi network is using what are considered insecure settings.

Apple recommended router settings here:

Recommended settings for Wi-Fi routers and access points - Apple Support

Apple devices display a weak security notice if the security settings on your router are not set to WPA2 AES or to WPA3.

How you change security settings on your router depend on your router brand/model. If you don't have instructions you can do an internet search for your router or if your router is supplied by your internet service provider, check their website.

Note that WPA3 is fairly recent and not supported by all routers buy WPA2 AES is an alternative that likely is supported

See especially “Weak security settings to avoid on your router” here.

Recommended settings for Wi-Fi routers and access points - Apple Support