This summary page is indicating that somebody on my local network (i.e. within my home LAN) is generating quite a lot of traffic to my computer. Most likely they are copying some files from their computer to mine.
So how do I know who's doing that? Well, I just go to the Hosts page!
Look at the first line, sorted by traffic. In the "Host DNS Name" I can see that all this traffic is generated by a PC named "sirius" - which tells me exactly who's doing this! (since it was me who picked that name for one of the computers in the first place)
Or, if you just want to know which nasty program is using lots of traffic, go to Processes: