To easily recognise if Meta Tags have been defined, then if the icon is black in colour then Meta Tags are undefined, and if it is orange then Meta Tags have been defined.
Defining your own Meta Tags allows to to create a much more comprehensive product title and description - That title and description are two of the elements used by Google to filter your pages in search results, so it's really a good idea to make them both as descriptive as possible.
If you look closely at the search results in Google you'll see that any words you typed in the search box are highlighted in bold type in the Title and Descriptions.. I've highlighted them with a red underline in the above image.
That means that Google has found and used those words to filter the search results, so from an SEO perspective, if your page Title and Description Meta Tags contain words appropriate to the product you are selling, your pages will figure reasonable well in searches.
Example... If you are selling 'Left handed widgets' then make sure that both the page title and description Meta Tags contains the words 'Left Handed Widgets'. You will get much better placement for that product in the search results if they do.
When creating Title and Description Meta Tags it's worth noting that the Maximum character lengths displayed for page Title and Descriptions in Google results are...
Page Title: 69 Characters (Including Spaces)
Description: 156 Characters (Including Spaces)
Title and Description text lengths exceeding the above are truncated, and are not displayed in the results.