They are little markers attached to data points in graphs to show the size of the uncertainty in the value being plotted. Say you have a graph of the temperature measured each day at nine am in the morning; your thermometer is only accurate to one degree Celsius, so you measure a value of 10 degrees, but the actual temperature might be 9 or 11. When you plot it on your graph you put a dot at 10 degrees and then draw little lines up and down from that point to show the interval the actual value is in. (I do hope that made sense; it is the sort of thing that is easiest to explain with a picture, but I haven't got one to hand!)
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branchandroot
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Date: 2014-10-09 09:58 pm (UTC)