A lot of customers forget to enable Cron and miss out on some important features.
They also get more frustration when things don't work.
There isn't anywhere clearly in the setup process that says you should switch on Cron- I think we should add it to make new users life easier! ( even if its just a page after setup that says you should set up the following post install)
Link to docs: ( and features dependent on Cron)
First thing first if you have ssh access to your server you have to set it up with this command:
sudo crontab -u root -e
And then add inside the file you'll open a line similar to this one:
* * * * * /bin/sh /var/www/<yoursite>/cron.sh
This will automatically execute your magento cronjobs
At the moment this is valid for magento 1.9, I think it should be alright also for 2 (you should look where the cron.sh is situated in magento 2).
Hope this helps.
moreover @grantkemp pointed to a correct solution in the case you're setting additional cronjobs to your system.