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!
Or, if you just want to know which program is using lots of traffic, go to Processes:
