When you’re writing—you usually have a few options for text alignment—at least, in MS Word, you can flush left, center, flush right, or justify. Justify in Word actually means left justify, so I’m going to call it that.
Basically, justifying text gives you clean left and right edges, which flushing or centering text gives you at least one ragged side. People tend to like justifying things because they think it looks better, but there are drawbacks.
To make every line of text exactly the same width, the computer will adjust the size of the spaces between each word. Sometimes this will result in ugly lines of open space running down your paragraph. We call these rivers.
To avoid these rivers, you can either flush your text left (my preference) or allow hyphenation, which dynamically cuts up your words to fit the column of text. You can see examples of all three here: