Why does it say weak security under wi-fi

A "Weak Security" notification means that your Wi-Fi router is not configured correctly. There is a new feature in iOS 14 that checks and displays a message if your router is not using the most up-to-date security. Note that your router has always had weak security, you just didn’t know about it until now. 

See this Apple support article for how to configure it correctly—>Recommended settings for Wi-Fi routers and access points - Apple Support, specifically the section headed Security. And don't forget to configure your range extenders (if any) also!

If your router is less than 5 years old you probably don’t need a new one. You need the manual for your router, which you can find on the manufacturer’s website. Referring to the manual:

• Using a web browser, log in to the router control center - you will enter either an IP address or a URL, typical values are https://10.0.0.1/ or https://192.168.0.1/ or some variation that begins with either 10 or 192.
• Navigate to the type of encryption; it will be either a series of buttons or a dropdown
• Change the setting to WPA2(AES), WPA2-PSK(AES), WPA2-Personal or WPA2-(AES/WPA3)
• Do not choose WEP, WPA/WPA2 or anything that includes WPA
• After you change it, on your phone go to Settings/Wi-Fi, tap on the i next to the network name and forget the network. Then reconnect.

You should only have to do this once.

Change the security protocol on your router.