图像滑块如何在附加代码中从一张幻灯片进展到另一张幻灯片？

I was given the following code which progresses from one image to the next (previous or next) in a slider. It worls fine and I inderstand that specifically the function cycleitems does the progressing. What I don't understand is the precise mechanism for moving the slide from currentindex to (currentindex plus or minus 1). Can you explain exactly what the statement I was given the following code which progresses