The simple answer: The side soundhole is a second soundhole (1.75" diameter) in the top of the guitar. It helps get the sound from the strings more directly to your ear.
Sometimes when you're playing in noisy environments or playing nuanced songs on a guitar with a traditional soundhole, they don't sound quite right. This is because the directed sound from the front of the instrument has farther to travel and bounce before it makes it back to your ear. The increased distance and environmental variables make it more likely that some frequencies will be "lost" or distorted.
The side soundhole is a great option for people who play alone, in small venues, amplified, or want the best exposure to the raw sound coming from their KLOS instrument.
Since the side soundhole offers sound another route to escape from the soundbox, it can reduce the volume substantially. If you're using the instrument for gigs, you'll probably want to amplify the instrument to get the best possible projection and presentation. We're working on finding a way to cover the second soundhole and regain that projection.
We also offer the second soundhole on our ukuleles.
Placement on a guitar
Placement on a ukulele