What is a wildcard virtual server

A Wildcard Virtual Server is special type of virtual server that have a network IP address as the specified destination address instead of a host IP address. Whenever the BIG-IP does not find a specific virtual server based on the client's destination IP adress, it tries to match a wildcard virtual server that have the designated IP address of 0.0.0.0. Then the BIG-IP system will forward the client's packet to one of the firewalls or routers. This is used to route traffic to transparent devices, like in your case, firewalls.

There are two types of wildcard virtual servers, default wildcard servers and port-specific wildcard servers. Default will have the port and address of 0.0.0.0:0 and port specific will have 0.0.0.0:80 for HTTP.

This is a great source that explains it further: https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/ltm-basics-12-1-0/2.html