There is a motivational speech which I'm having trouble understanding. Here's the phrase:
There are two days either side of a dash, make sure that dash is not empty.
What's the meaning of it?
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It's an allegorical reference to a tombstone inscription:
Between the date of your birth and the date of your death is all the rest of your life, represented visually by the dash between the dates on the tombstone.
It is encouraging you to "make your life matter".
This phrase itself isn't an English idiom, per se, but the allegory is widely known in the religious/inspirational community.
This tweet may be the version that you heard (thanks, @mahmudkoya), which makes more grammatical sense than your original quote:
One day it will be over. There will be 2 dates, either side of a dash. Make sure that dash is not empty. - Fearless Motivation
Some further references and variations: