Tip of the Day is a common feature coded into many software packages, operating systems, window managers, and websites.
In software packages and operating systems, the basic idea is that every day a new tip will be displayed showing the user how to use a new feature of the program. This can be as simple as telling the user how to save a file, or as advanced as providing tips on esoteric command line switches. Most power users turn this "feature" off in any and all software they use, as it is usually implemented as an annoying pop up window (and who really likes those). Less informed users often do not turn this off, but tend to just close the window with out even reading it.
Many websites also include a Tip of the Day. These tips are usually related to the site's main topic (a soccer site may have daily soccer tips, while a stock site may include daily stock tips). Some websites fail to update their tips, or abandon them altogether. Website tips are rarely implemented as pop ups, and will often be a separate page, or a small blurb on the main page of the site.
Tip of the Day calendars have been produced on many topics. Much like the old "Word of the Day" ones, they seek to impart a small bit of knowledge to the reader on a daily basis.
Try to take a bit of time to read your tips of the day whenever they are available, as you can never know too much about anything. | 
