Basic basic set theory. Please help. Simple answer will suffice.

[andrewkg]

Hello my name is Andy I'm in high school, and I have a bit of a confusion or lack of information. Ok so I have been reading a book on set theory, and I keep encountering … ,but up more in the use of a set. Like A,B … but again up more to the middle of the sentence. I feel dumb, I googled it nothing, I've been thinking about it for around 2 hours. I cannot sleep it is bugging me so much. Thank you in advanced!

 

[Simon Bridge]

I keep encountering … ,but up more in the use of a set. Like A,B … but again up more to the middle of the sentence

You mean you keep encountering three dots like this "..." or "$\cdots$"?
They are called "ellipsis" and stand for the words "and so on", and mean that things continue as implied by what went before.

So:
$A,B,\cdots$ means start going A B C etc and keep going in the same pattern ... so $\{ 1,3,5,\cdots \}$ would represent the set of positive odd numbers
$\{ 1,3,5,\cdots,27,29 \}$ would be all the odd numbers from 1 through to 27 inclusive.
They are written like that because it would be boring to list every single member.

In a sentence it indicates, informally, a longish pause - in a quotation, it means that part of the quote that goes there was deleted for clarity.
At the end of a sentence it indicates that the sentence has been deliberately left unfinished.

See:
http://www.mathsisfun.com/sets/sets-introduction.html
http://en.wikipedia.org/wiki/Ellipsis