Meaning
Lifespan, the length of one's life; age.
Literally: Arabic 'umr: the duration of a life.
How Iqbal uses it
Umr is the measure of a life; for Iqbal its worth lies not in mere length but in the intensity of selfhood and deed packed into it, so a short life lived greatly outweighs a long one drowsed away.
See it in the verse