It meant finding people who possessed food, and taking it from them. In an economy built around agriculture, that meant peasants and their communities. In most cases, it meant taking it by force or the threat of force. Medieval armies, generally speaking, lacked the logistical ability or the economic base to supply themselves any other way; they couldn't transport it from their core areas, and paying for it would have been both ruinously expensive and unreliable; what if the peasants didn't want to sell?

hi! hopefully one of the military experts from that period will jump in here with specifics, but meanwhile, for a basic idea of "foraging", check out comments in these threads

• Question about the use of foraging by armies. - quick tl;dr from /u/rittermeister: "Foraging parties don't [hunt] for roots and berries; they find locals and rob them of their food and valuables."

• How did medieval armies supply themselves?

• How did generals feed huge armies in the field in ancient and medieval times?

• How were ancient and medieval warriors able to stay hydrated on the battlefield? Did they bring some equivalent of canteens onto the battlefield?

• Were knights/Medieval soldiers as brutal and remorseless as depicted in 'Game of Thrones'?