objectClass
Explanation:
The objectClass attribute of an LDAP object defines which other attributes can be set for the object.
The objectClass attribute is a multi-valued attribute that specifies one or more object classes that the object belongs to. Each object class has a schema that defines the mandatory and optional attributes that the object can have.
The objectClass attribute is required for every LDAP object and determines the nature and characteristics of the object. References:
Understanding the LDAP Protocol, Data Hierarchy, and Entry Components
LDAP Object Classes

listen
Explanation:
The listen directive in a Nginx server configuration block defines the TCP ports on which the virtual host will be available, and which protocols it will use. The listen directive takes one or more parameters, such as the port number, the IP address, and the protocol name. For example, the following directive tells Nginx to listen for HTTP requests on port 80 on all network interfaces:
listen 80;
The following directive tells Nginx to listen for HTTPS requests on port 443 on the 192.0.2.1 IP address:
listen 192.0.2.1:443 ssl;
The following directive tells Nginx to listen for both TCP and UDP traffic on port 53 on the 192.0.2.2 IP address:
listen 192.0.2.2:53 udp tcp;
The listen directive can also take some options, such as default_server, which specifies that the server block should act as the default server for the given port, or backlog, which sets the maximum number of pending connections for the socket.
References:
Server Block Examples | NGINX
NGINX Docs | Configuring HTTP Servers
Which directive in a Nginx server configuration block defines the TCP ports on which the virtual host will be available, and which protocols it will use?

Explanation/Reference: https://www.digitalocean.com/community/tutorials/how-to-protect-ssh-with-fail2ban-on-centos-7