Tablet says "SD Card Mounted"

Though I've never seen this kind of notification sticking (or even appearing): It's obviously informing you that your SD card was mounted. Note that almost all recent Android devices come with an internal-sd card – so even if you have no external card, there's still one that should be mounted.

The other part of your question is a general one on notifications. First, there are two kind of notifications: the best known are those just giving you a one-time information, and you can simply "swipe them out". The other kind are so-called "ongoing notifications", and they "stick" (i.e. they cannot be "swiped out"). The only way to get rid of them is "silencing" the app giving them. Long-pressing a notification gives you a menu to do exactly that: telling the system you don't want this app giving you notifications.