In typography, “to kern” means to adjust the spacing between two adjacent letters. It can also mean to squish two letters together so they overlap to avoid ugly white spaces. Almost everyone is exposed to kerning, but probably few pay attention. It's an important design tool used by professional graphic artists (including book designers) and amateurs should learn to kern, too.
Compare the normal and kerned versions of a book title of mine:
InDesign can provide automatic kerning to adjust space within specific letter pairs. With Word, you manually condense the space between letters. It’s a lot of work that few folks bother with. I do it for book titles and some chapter names and subheads—but not for body text.
The “adult” software packages can cost as much as $800 and can take a long time to learn how to use properly.
On the other hand, most self-publishing writers already own Word and know how to use it. They can quickly learn how to use some of its often-untapped power to produce a nicer book.
If you are using Word, you can kern by selecting "character spacing" within the "font" section. Experiment with different settings for different parts of the word. Look at "professional" books and magazines and even product packages for inspiration.