What does 'current channel' mean in Yosemite wireless diagnostics?
Wi-Fi has multiple channels, which specify what frequency the network is communicating on.  On the 2.4GHz frequency, channels 1, 6, and 11 are non-overlapping channels, and all the others overlap on these channels.  (On the 5GHz frequency, the channels are spaced out to avoid overlapping).  You can think of channels as an offset value from 2.4GHz.

Current channel is the Wi-Fi channel your router is advertising and the channel your computer is connecting to.  You might get better performance by using either a 5GHz channel if your Wi-Fi router supports it, or one of the non-overlapping ones on the 2.4GHz.