It means it is about the same distance from Fair Isle to Sumburgh Head as from Fair Isle to North Ronaldsay, and tells you what those respective (almost equal) distances are.
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It is roughly the same distance from Fair Isle to Sumburgh Head as it is from Fair Isle to North Ronaldsay.
From Fair Isle, you would go about 38 kilometres (24 mi), traveling to the northeast, to get to Sumburgh Head on the Mainland of Shetland.
From Fair Isle, you would go about 43 kilometres (27 mi), traveling to the southwest