There an image of 4 wheels, the top 2 are tilted and there's a cross between the top set and the bottom set. I pressed it once and it made a noise that was less than pleasant and the car was revving weirdly. It took me 30 minutes to turn it off because pressing it again didn't work. I'm driving a 2006 Daihatsu Terios
That button is for the centre diff-lock.
The "normal" 4wd setting is the engine power is biased 60/40 to front / rear.
Once traction is lost, and any one wheel spins you are stuck.
Locking the centre diff "fixes" the drive to front and rear so if the rear has traction you will move, or if only the front has traction, again you will move.
However, if one front and one rear spin you are now stuck...
This should not be operated at speed on the hard-top and only in slippery conditions.
If it makes lots of noise then get it checked.