What does 'let dry lightly between waterings' mean?

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Most horticultural terms are so vague they're meaningless. My best guess is "don't keep completely wet."

How to water orchids varies with the immediate growing environment. I haven't grown the Isabella. Epidendrum parkinsonianum does best kept continually moist during the warm days, cooler nights growing season so long as there is plenty of air at the roots. In the long dry season, with cool days and cooler nights, there is often dew at night, so it is not dry for long. It grows hanging down off trees. Many people mount it if they can keep up with daily or almost daily watering.