Sorry for this stupid question - but what does subtitle function means in a DVD player? Based on some readings in internet about player reviews, some reports say that some players don't support "subtitle". Those players are criticized heavily by not supporting this function. Is this the same "subtitle" on movie? But if it is, then how come I can still see the subtitle button on the remote (the player was reported not supporting subtitle, though)? Or does it mean something else?

The Subtitle buttons turns on and off subtitles, if the disc has them. I have never heard of a DVD player that doesn't support this function. I'm pretty sure it's a mandatory part of the DVD spec.