The official Treknobabble explanation, seen in the TNG Technical Manual and other official media, is that most of the stuff on the holodeck is holographic and can't be carried outside but some stuff (like food and drink) is replicated matter you can really eat and drink. The "mostly holographic but sometimes it's replicated" explanation also covers the occasional on-screen inconsistency like when someone in an episode threw a snowball out the holodeck doors into the hall, or when someone fell in holodeck mud and left footprints in the corridor as they left.