Yep
It's for when you have an external video source such as a vcr/cable box, or satellite tuner that is feeding video to your capture card via an RF / composite cable on a single channel.
For example, your box is tuning to your desired cable channel. But is outputting on channel 4 to your tv. You would want your capture card to stay locked on channel 4 in order to see the output of the cable box, and then send tune commands via IR or serial cable to the cable box to change the channel there.
So, checking the 'lock tuner' box will keep your _capture_ card set to one specified channel, and instead change the channel on the external box.
Without it, btv would try and change the channel of your capture card instead.
Um. That sounded like it made sense but I'm not sure it was necessarily clear...?