It most likely means that you have a number of slates that have been strapped back in position after coming loose in the past. They are refixed by nailing a copper, lead or aluminium strip known as a Tingle to the lat with the tail of the strap protruding below the course of slate and then a slate is slid back up into position and the end of the strap is bent back over the slate to keep it in place. In general it may mean that the roof is of such an age that the nails have perished and this causes the slates to slip. I woulkd advise getting a roofer to give it an appraisal and a quote for any work it may need.
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