Fuzicat,
One Finger Accord are accords you play as it says with only one finger.
When the auto acomp is on and if you press a C (that is the white key before 2 black ones) on the left side of the keyboard, thus left of the splitpoint you will hear the full C-accord what normally is played with 3 or 4 fingers. That is C-E-G or C-E-G and the C one octave higher.
The same for ALL the other keys left of the splitpoint (that is by default the F# or Fis the first of 3 black keys left of the middle or central C in the middle of the keyoard).
Hope you understand it now. At first it looks maybe complicated but you will learn to use it very soon.
Good luck with your new keyboard.
DickR