True random number generator vs. pseudo random number generator? Is the former possible to implement in C++?

Yesterday I visited a science exposition and spoke to some people that did research into random number generators and they have apparently designed a true random number generator (RNG). That really made me interested as I thought these sort of things are just not possible. They said that RNGs are very hard to test, but there are many ways to test them and they used one of those and it passed those tests. And also apparently later on the true RNG things could be implemented as a library for a language like C++.

So is it safe to say that a RNG is TRULY random if it passes the tests? And can someone please explain how can one achieve true randomness? Can we somehow generate true random numbers in a written program?

Thank you