What does detecting array mean?

If this happens at startup, before Windows loads, it's probably that your motherboard has a facility to utilise a RAID ARRAY, (several hard drives wired together). Some motherboards report the presence or absence of an array as part of its startup procedure. It can be alarming, but it only means that it did not detect an array and was just telling you so. After a couple of seconds the message should disappear and Windows will load.