My wifi setting says weak security.. how do I resolve n why is this happening?

Change router settings for wifi security WPA2/3

Perhaps this link will help...

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

Basically the phone is letting you know that the security mechanism of your router is not secure. It doesn't mean that there is a breach or anything like that. You can ask you internet service provider to change the settings of the router and use a newer security protocol but it's not a big deal. You could ignore it.

It is still secure! Probably on WPA-psk, but its just a bit easier for a determined neighbour with the right kit to use your wifi but still unlikely.