A nosebag is a bag, usually made of canvas. You put some feed, usually grain, in the nosebag and then attach it to the bridle or halter.
Nosebags are most often seen in situations where horses are asked to work long days without access to grazing, such as carriage horses.
They may also be used when feeding horses at pasture as a way of making sure a subordinate horse still gets the correct amount of grain, or if giving medication.
This Turkish horse is enjoying a lunch break with his driver during the harvest in Cappadocia, Turkey. Source: Yelkrokoyade via Wikimedia Commons.