Zarra – means the smallest part of something, an atom, a grain.

Here is a beautiful ghazal written by Sudarshan Faakir that includes this lafz:

aaj ke daur mein ai dost ye manzar kyun hai
zakhm har sar pe har ik haath mein patthar kyun hai

In today’s times, my friend, why is the scenario such that;  There is a wound on every head, and a stone in every hand

jab haqeeqat hai ke har zarre mein tu rehtaa hai
phir zameen par kahin masjid kahin mandir kyun hai

When the reality is that you exist in every little particle of this world; Then why in this world, is there a mosque somewhere and a temple somewhere

apna anjaam to maaloom hai sab ko phir bhi
apni nazron mein har insaan sikandar kyun hai

Every person knows their eventual outcome, Yet, in their own eyes, why does each person think himself to be great (an emperor like Alexander the Great)

zindagi jeene ke qaabil hee nahin ab “Faakir”
varnaa har aankh mein ashkon kaa samandar kyun hai

Life is not worth living only, Faakir (name of poet);  Or else, why is there an ocean of tears in every eye

Link to the ghazal sung by Jagjit Singh is here: https://www.youtube.com/watch?v=e3SrDCkb4Bw