What is Effective security rules? · Ultra Transcenders glossary

Network Watcher feature showing the merged subnet and NIC NSG rules that apply to a network interface; it does not test any traffic.

In the Ultra Transcenders books

Each book explains Effective security rules in context, with comparison tables and the common traps.

Network Watcher
Collection of Azure network troubleshooting tools: next hop, IP flow verify, packet capture, Connection Monitor and flow logs, among others.
Subnet
A segment of a VNet's address space from which resources receive private IPs. Azure holds back five addresses per subnet (the first four and the last), leaving 251 usable in a /24 and three in a /29, the smallest IPv4 subnet.
NIC
Network interface card, the network interface of a VM; NSGs can be associated with it.
Default security rules
Built-in NSG rules at priorities 65000-65500: AllowVnetInBound, AllowAzureLoadBalancerInBound and DenyAllInBound, plus AllowVnetOutBound, AllowInternetOutBound and DenyAllOutBound. Removal is impossible; custom rules at 100-4096 take precedence.
APPLY
Evaluates a table-valued expression for every row on its left, inside FROM. Think of OUTER APPLY as a left outer join and CROSS APPLY as an inner join.