A "Net" is a set of interconnected points like ground. Nets get names from 2 primary places one is the symbols connected. For example a ground symbol is enough to create a net called 'ground'. The second source is the pin numbers assigned to an IC or other part.In the example I pointed out the error was in not editing the net created by interconnecting those 4 symbols, 2 caps a crystal and a processor.
Finally there will from time to time be net names you will create or modify to fix/add/modify circuit connectivity (the netlist).
Bob