Nano is term with expresses very small or very little. When counting or measuring it means one billionth. So one nanometre is one billionth metre or 1nm=0.000,000,001 m.
Nanobacteria are the smallest known bacteria, said to be about 50 und 200nm big. They have not been seen before, as common microscopes were not able to show such small structures. When the technology was imporved during the 1980s, smaller structures were found.
There is still a discussion if nanobacteria exist. The porblem is, such small animals do not contain very much molecules, so it is hard to talk of live. Live would mean movement, consumation of energy, and reproduction.
Why are this nanobacteria called cave live? To be honest, they are not in the classical sense of the term. But they are also called extremopiles, which means they adapted to extreme environments like hot springs, underground fissures, dark and cold places and so forth. It is logical to believe that there are a lot of them in caves.