Explain the keys please
Is this also for your exam?
Within the context of microcontrollers in general, nothing, except that mem may indicate something related to memory.
If you are trying to research something for a project, please link us to the thing you're trying to understand.
If you're a student trying to get easy answers for your test as another has suggested, shame on you! Go study the book some more - this is a facinating subject area with abundant career opportunities, and you should be very thankful that you're getting a class on it; I'd have killed for a class on microcontrollers.
DrAzzy:
If you're a student trying to get easy answers for your test as another has suggested, shame on you!
It absolutely was not an "if"!
See here!
It seems crazy to try to get exam answers off the internet.
But I even had a colleague like this, he was going to internet forums, asking "how do I write a function to do X?", then "how do I write a function to do Y?" and trying to paste the results into a program.
Needless to say, his program didn't work. I had to laugh at the sheer audacity of this!
Many questions like this make me question the state of students today 
Some of them make me question the state of teaching ("Fanout of an Arduino pin"??!! In the absence of multiple choice or a clear (but mostly wrong) in-class decree, that's a really AWFUL question... And "exex"?)