Audio/Video Output Setup - Match video framerate

Under Audio/Video Output Setup --> Video Output --> HDMI (auto), Auto, … … , it asks “Match video framerate” On/Off .   What does it do?  I haven’t had any problems with playback if this is on or off.  What is the default setting?

Thank you.

If you’re watching 24 fps material, it would switch your HDMI mode to 1080p24.