[HarryT]

Becca, this is clearly explained in the tutorial that was linked to earlier:

There are cases where you might want to have text appear in the output only if a field is not empty. A common case is series and series_index, where you want either nothing or the two values with a hyphen between them. Calibre handles this case using a special field syntax.

For example, assume you want to use the template:

{series} - {series_index} - {title}

If the book has no series, the answer will be - - title. Many people would rather the result be simply title, without the hyphens. To do this, use the extended syntax {field:|prefix_text|suffix_text}. When you use this syntax, if field has the value SERIES then the result will be prefix_textSERIESsuffix_text. If field has no value, then the result will be the empty string (nothing); the prefix and suffix are ignored. The prefix and suffix can contain blanks.

Using this syntax, we can solve the above series problem with the template:

{series}{series_index:| - | - }{title}

The hyphens will be included only if the book has a series index, which it will have only if it has a series.

Notes: you must include the : character if you want to use a prefix or a suffix. You must either use no | characters or both of them; using one, as in {field:| - }, is not allowed. It is OK not to provide any text for one side or the other, such as in {series:|| - }. Using {title:||} is the same as using {title}.

If you can clarify which part of this you're having trouble with, I'll be happy to explain further.