When I try to run the GUI, I get these errors:
./monero-wallet-gui: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `CXXABI_1.3.9' not found (required by ./monero-wallet-gui)
./monero-wallet-gui: /usr/lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.21' not found (required by ./monero-wallet-gui)
What are these .so.6 files?
Those errors mean that you are trying to run a binary which was not built for the same system libraries you are currently running. That is, monero-wallet-gui was built against versions of libstdc++ (the C++ standard library) and glibc (the GNU C library) which are not compatible with the ones you have on your system.
Your options are to either wait for the next release of the GUI binaries, which will be built to be compatible with a wider variety of machines, or to build the GUI yourself, on that machine.
those .so.6 files are shared libraries. The ".so" suffix is traditionally used for them, and is short for "shared object". The .6 is a version number, signalling the 6 version of the library with incompatible changes. Here, you do not have to care about those versions, since you likely won't try to build compatible libs.
