Daily Job
This job must run each day after midnight. It resets all statistic of the previous day, sends daily emails and many more. If you are interested what exactly it makes, take a look to Administration => Reset/rebuild. Note that it's very important to run the daily job immediately after midnight to have the statistic as accurate as possible.
There are 3 different ways to run this job:
Automatically - Check field "Run the regular jobs automatically" in your Configuration. This is a very easy way but it needs additional system resources and it is not recommended. You should never use this way if you have more than 50,000 impressions/day because it may be too difficult for your server.
By Crontab (Linux) or Task Manager (Windows) - it's the best way but there is needed a little more setting. You can read below how to set a Crontab job on your server. If you use this way, never check the field "Run the daily job automatically" in the Configuration.
Manually - You can run this job manually in the Administration. Note that if you run it in other time than after midnight, it can damage the daily and monthly statistic.
Using Crontab
If you didn't use Crontab before, please go to http://webmasterworkshop.com/guides/crontab_guide.shtml to a simple info about it.

You will need to figure out if you will be using lynx or wget or GET to perform the Cron tasks. These are programs that are commonly available on Linux and Unix servers, but not always. You will need to locate one of these programs on your server. If you do not know where they can be found, contact your server administrator to get that information. You will need the full path to the program (ie. /usr/bin/lynx).
Note that if you set up in your Configuration that time on the server is different from a real time, you must move also the Daily Job. For example if there is +2 hours difference, you have to run the Daily Job after 2 am.

You have to add one of the following lines to your Cron tasks.
It runs this job daily at 0:01 am, it's very important to run them immediately after a midnight.
If you will be using wget, the line is:

    1 0 * * * /path/to/wget --spider -q 'http://www.yourdomain.com/rebuild.php?word=yoursecretword'

If you will be using lynx, the line is:

    1 0 * * * /path/to/lynx -dump 'http://www.yourdomain.com/rebuild.php?word=yoursecretword' >/dev/null

If you will be using GET, the line is:

    1 0 * * * GET 'http://www.yourdomain.com/rebuild.php?word=yoursecretword'

You must replace "/path/to/wget" or "/path/to/lynx" with real path to wget or lynx.
You also must replace "http://www.yourdomain.com/rebuild.php" by a real URL of your rebuild.php file. Also replace "yoursecretword" by your secret word you have entered in the Configuration.
These are only examples. You may want to change the times if you are familiar with Cron.