Queues allow to defer time consuming tasks, such as sending an email, until a later time. This way performance of some user interactions on the site can be improved.
Queues are fully optional and do not provide any additional functionality. If disabled, actions that could be queued will be performed instantly instead.
To setup queues on the server you will need access to command line to install required dependencies and start the queue worker process.
There are several queue methods you can select from admin > settings > queue page. All of them are identical functionality wise, but Redis is recommended as it will be easiest to setup and is generally the most popular option.
After you've selected the queue method, you will need to start the queue worker using this command:
php artisan horizon
Ideally you should also configure a process monitor (supervisor, systemd etc.) to monitor the php artisan horizon command and restart it if it exits unexpectedly or server is restarted.