The phrase "gham-e-dehar" (غمِ دہر) is an Urdu expression where:
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Gham (غم) = sorrow, grief, or pain
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Dehar (دہر) = the world, time, or the age (often used poetically to mean "the world or its troubles")
So, "gham-e-dehar" means:
"The sorrow of the world" or "The worldly troubles/pains of life."
In Faiz Ahmed Faiz's poem, when he says:
"Tera gham hai to gham-e-dehar ka jhagra kya hai"
He means:
"If I have your sorrow (to occupy me), then what do the rest of the world’s sorrows matter?"
It reflects the classic romantic theme where personal love makes all other worldly concerns seem insignificant.