What is bleeding of steam in thermal power plant?

Bleeding of steam means Extracting steam from Turbines for heating the feed-water.

Normally, there are High Pressure Heaters & Low Pressure heaters in Condensate cycle. Steam bleed out from the turbine gets into these heaters and heats up the feed-water (generally termed as Sensible Heat Addition).

The steam that was bled can be re-used as condensate in Deaerators.

Steam Extractions although reduce net work output but helps in overall gain in efficiency in terms of Reduced Fuel Consumption.