Espresso channeling occurs when water finds a small path instead of flowing through the entire coffee puck evenly. When water finds a weak point in the puck, it will start eroding the bed and create a channel. That path will have less resistance and let more water through making an uneven flow.
As a result, the coffee extracted near the channel will be over-extracted and the rest of the puck will be under-extracted. This results in espresso with an uneven flavour profile with the shot being acidic and bitter at the same time.