Can anyone please breakdown what is going on in this script?

$node = $appConfig.configuration.connectionStrings.add | Where-Object {$_.name -eq $dbName} $node.connectionString = $updatedConnectionString 

1. Where did add come from?
2. What does $_ stand for here?
3. How is $node.connectionString assigning to the attribute when there is no property on $node called connectionString?

Lastly, any resource that can help me understand this and not ask stupid questions will be really appreciated.