LEDE system is composed of many different programs running all the time called daemons or services.
The default ones are already set up in a standard installation, but you may want to change their configuration or add new ones with packages.

service will print a list of available services

service service_name will print a list of available actions for a service

service service_name action will execute that action on that service

Technically speaking, the service command is just a wrapper for initscripts found in /etc/init.d.
The above commands are functionally identical to the following ones (respectively), they are just shorter and easier to remember.
ls /etc/init.d/
/etc/init.d/service_name
/etc/init.d/service_name action

Click on System, then on Startup to land in the page listing all the available services and buttons to execute actions.

The default actions supported by most services are: