A stitch pattern dictionary is a book of stitch patterns such as you would need if you wanted to design your own garments. For instance you could take a basic sweater pattern and use a lace stitch or a textured stitch, cables, etc. that you pick yourself.
Stitch pattern dictionaries sometimes contain some how-to information but they are usually not the best source for that type of information.
The absolute best stitch pattern dictionaries in my opinion are the ones by Barbara Walker called Treasury of Knitting Patterns, vol. 1 & 2. They are not inexpensive but they are by far the most extensive.
The Harmony Guides are a reasonably priced alternative if the Walker books are either too expensive or too advanced for your needs.