It is shorthand for:
netsh int ip reset resetlog.txt
netsh i i r r
The last ‘r’ being the “log file” that you really don’t care to read.
What does it do? It hits your TCP/IP stack with a sledge hammer. And forcefully rebuilds it via a rewrite of two Registry keys:
SYSTEMCurrentControlSetServicesTcpipParameters
SYSTEMCurrentControlSetServicesDHCPParameters
This has the same effect as removing and reinstalling TCP/IP.
Please do this command in an Administrative command prompt
http://www.thewindowsclub.com/how-to-run-command-prompt-as-an-administrator