Meaning
Lustrous, shining, radiant; also curling and glossy (of hair).
Literally: Tab (brilliance, also twist) plus dar (possessing).
How Iqbal uses it
In the Khudi quatrain Iqbal bids the radiant, curling lock be made yet more tabdar, using the gleam of the beloved's tress as a figure for a self polished to ever greater brilliance.