C++ has a long history, going back to 1979 when it was called C with Classes. As it evolved, it has been standardized as an international standard by the ISO. There have been three standards: C++98 in 1998, C++03 in 2003 and C++11 in (you guessed it) 2011 and a technical report (C++TR1 in 2007). C++11 was refereed to C++0x or C++1x prior to it’s release and you’ll see that in older articles. The three standards have improved the language and there is a new standard in the works that’s currently refereed to as C++14. The Wiki C++ article has more details if you care.
UE4 is limited to a subset of C++11 that’s supported by Microsoft C++ 2013. The other two compilers that UE4 supports (GCC and Clang) both support full C++11, so the limit is MSVC++. Even so, the UE4 code base isn’t pushing the new features of C++, so you really don’t need to worry about them, unless your making change to the Engine source itself. You can use them as you need them, since it won’t affect portability. For example, I use lambdas a lot as they’re very useful for events and callbacks.
For you, I’d recommend focusing on the core c++ features and not worry to much about C++11.