In my experience, the Busy Channel Lockout or Busy Channel is the name for a feature which is completely different from what Cranky Emu describes.
The Busy Channel Lockout prevents transmission on a frequency which is, or appears to be, already in use.
Normally, you'd be able to hear the other side which is transmitting using your receiver, and avoid transmitting until the channel is free.
However, with more complex squelch systems, such as CTCSS and DCS, becoming more common, it can happen that your receiver is receiving another signal, but is muted due to squelch settings.
This might lead you to believe that the channel is free and that you can transmit on it. If you do transmit on it, you could create interference to the transmission already in progress. Do note that some radios, especially those without screens, might not have any way, other than audio, to signal that there is another squelched user on the channel.
Additionally, depending on the implementation, the busy channel lockout feature might also work even if the CTCSS or DCS are not in use. It might prevent radio from transmitting as long as there's some signal on the frequency. This can cause issues which prevent the radio from transmitting in case where the radio is picking up interference which is not related to communication.
Also a bit of background:
In some jurisdictions, commercial users might receive licenses for frequencies shared with other users, but with each group having different squelch settings. With receive squelch, and busy channel lockout, it's possible to create a feeling of privacy, in sense that members of other talk groups won't be able to just listen in to the traffic which is not from their own group.
Additionally, in some parts of Europe, the 2 m amateur band, and the 70 cm amateur band are still used for analog cable television and digital to analog set top boxes. The cable TV infrastructure might radiate, creating large number of carriers which might break squelch of radios and also trigger busy channel lockout.