Don here from Shopify!
In essence here meta description and SEO description are the same thing, though if you have not added an SEO description yourself or edited it in your admin then the meta info will auto-populate based on your product title and description.
You can read a bit more about meta fields in your products CSV work here, but in short, if this field (SEO title or description) is left blank then the meta title or description will be created from your product title and description in your Shopify admin.
Hope that clears it up, I can see where the confusion would arise alright!
Feel free to reach out to us using our support site any time you have some questions for us. :)
All the best!