Why does it say weak security on my WiFi on iPhone

Because you have insecure settings on your WiFi router.

Change the settings.

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

It's because the wifi you're connected to uses an outdated technology or encryption (such as WPA or TKIP) which are not secure anymore actually.

I suggest you connect to your router admin panel via your browser and change the wifi technology that is used (you can do that on most routers). Personally, the ip adress of my router is 192.168.1.1 but it can vary depending on your internet provider. I suggest you just try it with this ip and if it isn't the correct one, just google it to find the right adress.

example of my admin panel

If you have any questions, feel free to ask :)

(PS : If it's not your wifi, i'd suggest that you don't connect to it anymore)