Most of the strings on an acoustic grand piano are grouped in threes, with each group being tuned to the same note. When played normally, the hammer strikes all three simultaneously, giving a full, bright sound. When you depress the una corda pedal or soft pedal as it's more commonly known, the entire piano mechanism (piano action) moves to the right, so the hammer hits only two of the three strings.
The resulting effect is a slightly softer note, subtly muted and somewhat less bright in tone. It's a very subtle change in the sound and can be described as taking the 'edge' off the sound.
Of course, as your Roland Digital Piano has no strings, your piano faithfully recreates this feature, which can sometimes be missed if you are unsure what you are listening for.