An older Wi-Fi scrambling method that has been replaced because it is weak.
Like a patched-up old lock that was only meant as a stopgap.
A small office had one old printer that only worked with the old Wi-Fi mode. That setting weakened the whole network until the printer was replaced.
Network covers the managed firewall and switches that sit behind your provider's modem.
Should I turn it off?
Yes, unless an old device needs it. Replacing that device is better than weakening the network.

Every WILDCARD card exists to solve one named problem. Find yours.