A non-proportional, or "monospaced", or "typewriter" font, is one where all of the letters take up exactly the same amount of space on screen: a capital "W", a lower-case "i" and a space are all equally wide. The Courier family of fonts is commonly used for this.
A proportional font is one where each letter takes up just the amount of space it needs, so that a capital "W" is much wider than a small "i".
Unfortunately, the different sizes of the letters in different proportional fonts means that it's not possible to line up letters consistently: a "W" may be equivalent to three "i"s in one proportional font, and to four "i"s in another. This means, for example, that it is not possible to use a proportional font to format plain text tables or poetry correctly--lining up the spaces and words using one proportional font will cause it to look skewed using another.
You should always look at PG texts in a non-proportional font, even if you prefer to work mostly using a proportional font, because readers and automatic converter programs will assume that you meant to your text to be viewed using a non-proportional font.