There are three fields in this tab, of which only two are important:
The Meta Title field will be used by search engines to display our product. Use your most important keywords at the beginning. This title will be used by the search engines as the heading of your product listing. Create an informative title that describes your product and do not use more than 70 characters.
The Meta Keywords field was important for search engines in the past. The more keywords you put in, the better it was. But almost all search engines are ignoring this field nowadays and you may just as well leave it empty.
The Meta Description is important, because this will appear in the results of the search engines again. Keep it short and attractive though! Unfortunately, Magento doesn't assist you here, but generally you could say that the search engines will only show the first 160 characters. The Meta Information tab looks like the following screenshot:
