Choke occurs when your horse eats something that blocks the tube that connects the mouth with the stomach (the oesophagus). This happens when a horse eats to greedily, does not chew food properly because of teeth problems, eats odd shaped lumps of food eg carrots and apples or when unsoaked food like sugar beet is fed.
Vets usually administer calming drugs like tranquillizers and spasmolytics which stop the wall of the oesophagus over reacting. When the oesophagus stops trying to force the food down quickly it usually slips down gradually of its own accord.
If the situation is more serious your vet might try to pass a stomach tube to push the material further down. If the food does not pass down in 24 hrs the vet may pass water down the stomach tube to try and help.