Not stupid at all! Honestly, if I tell you that it took me a bit of head scratching to work out the MS = manuscript, will that make you feel any better? It should ;)
YA = young adult, unless there's another YA that I'm not familiar with (that is also a DISTINCT possibility).
Comp = comparative. So rather than necessarily comparing yourself to an author, it's more about giving agents a sense of the flavour of you book. So for example, I was chatting to a friend earlier who is writing a YA horror, and without knowing anything other than that, joking suggested her comp titles would be 'IT' meets 'Sweet Valley High'.
And that's kind of why it's tricky, because the advice is 'pick something new'. Although the advice is also along the lines of, if you're writing about a sci-fi monster and it fits, it would be stupid to ignore 'Frankenstein'. So it's a bit of a tough one. Especially since you also shouldn't be using mega-best seller either.
I am VERY happy to be corrected by someone more knowledgeable than me, especially since I'm also rather new to this whole thing and am trying desperately to figure it out.
Hope this helps? If it's confused you even more, I apologise!