Serial

The serial launcher runs each job consecutively, just like core cron would. The default settings for the serial launcher can be configured at /admin/config/system/cron/launcher/serial

Options

Job lock timeout: The amount of time in seconds a lock should be kept on a running job. If a job dies and the lock isn't released automatically, this is the amount of time that will pass before the lock is released. This option can be overridden per job.

Maximum execution time: The amount of time in seconds PHP should be allowed to run during a cron run. This is for the total time of all jobs during a cron run.

Maximum number of launcher threads: The maximum number of cron runs to run concurrently.

Run in thread: Determines which thread a job should run in. This option can be overridden per job.
For concurrent cron runs, the serial launcher needs to be instructed about this.
Example for 3 threads:

Normal (wget)

* * * * *      /usr/local/bin/wget -O - -q -t 1 "http://example.com/cron.php?cron_key=1234567890abcdef&thread=1"
* * * * *      /usr/local/bin/wget -O - -q -t 1 "http://example.com/cron.php?cron_key=1234567890abcdef&thread=2"
* * * * *      /usr/local/bin/wget -O - -q -t 1 "http://example.com/cron.php?cron_key=1234567890abcdef&thread=3"

Drush

* * * * *      /usr/local/bin/drush cron-run -uri=http://example.com --root=/path/to/drupal --options=thread=1
* * * * *      /usr/local/bin/drush cron-run -uri=http://example.com --root=/path/to/drupal --options=thread=2
* * * * *      /usr/local/bin/drush cron-run -uri=http://example.com --root=/path/to/drupal --options=thread=3

Concurrent threads are not possible with poormans cron. Poormans cron will preferably run in thread 1.

Poormans cron keepalive: If enabled, the poormans cron will, after completion of a cron run, issue an http request in order to trigger poormans cron again. The http request will not be issued more often than once a minute. The http request will be issued to $base_url unless overridden by the variables: ultimate_cron_poorman_url_options and/or ultimate_cron_poorman_http_options.