What they are
Keywords are a list of important descriptive words or phrases separated by a comma and a space up to 255 characters in length.

How they are used

• Searching When a user does a keyword search, their query is matched against all keywords and inside of keywords, for example: if a user searches for 'cat' their results will include items with the words 'cat', 'catalogue', 'caterpillar' etc. in their keywords.
• Page indexing The keywords and description for a given item are what will be seen by search engines like google, using descriptive, properly formatted keywords will ensure you get the most exposure.

Choosing good keywords
Your keywords should include things like the number, gender and species of the participants, kinks involved such as bondage or watersports and any other pertinent information (i.e. if the image is mostly red, 'red' would be an appropriate keyword). Also be sure to include any abbreviations of keywords, like 'ws' for watersports and 'bd' for bondage & domination.

An Example