The chikou span is simply today's close drawn in the past. Traditional Ichimoku rules require it to be clear of the price action from that earlier period before a signal is considered valid, on the argument that overhead supply from that time has been overcome.
It is the clearest illustration in any indicator of the difference between display and information. Nothing about the line is predictive; it is a visual way of asking whether price is above where it was twenty-six bars ago.
It also causes real confusion in backtesting. Because the line is drawn in the past, naive code can appear to use future information. Any system referencing the chikou span must align the comparison carefully or the results will be fictional. See repainting.
Related: ichimoku-kinko-hyo, senkou-span, repainting, backtesting, rate-of-change