The product meta description is an HTML attribute that provides a brief summary of your product. Search engines such as Google often display the meta description in search results, which can influence click-through rates.
Google recommendation on meta descriptions length: Google generally truncates snippets to ~155–160 characters.
The importance of meta descriptions
Generally, meta description tags, while not tied to search engine rankings, are extremely important in gaining user click-through from SERPs. These short paragraphs are a webmaster's opportunity to "advertise" content to searchers, and searchers' chance to decide whether the content is relevant and contains the information they're seeking from their search query.
A product meta description should intelligently (read: in a natural, active, non-spammy way) employ the keywords that product is targeting, but also create a compelling description that a searcher will want to click. It should be directly relevant to the product it describes, and unique from the descriptions for other pages.