WPA2 and WPA3 are encryption standards for your Wi-Fi. If your router is set to an older (and weaker) encryption standard you will get a weak wi-fi security warning on Apple devices. When that security is weak, someone within range of your wi-fi could potentially intercept your wi-fi signal. The following also has information that may help: https://consumer.ftc.gov/articles/how-secure-your-home-wi-fi-network
I have a router that supports WPA3 and I use that -- but it isn't an issue I personally would obsess over if I didn't have that.
To eliminate the weak security warning, change the security settings on your router to WPA3 Personal, WPA2/WPA3 Transitional or to WPA2 Personal AES. How you do that varies by router. If you don't have the instructions for your router, do an internet search for instructions for your router brand/model or if your router is supplied by your internet service provider, check their website or customer support. Also see: Recommended settings for Wi-Fi routers and access points - Apple Support