In simple terms, choke in horses is a result of some obstruction within the esophagus which then results in items such as food or even water from being unable to get into the gut. One thing that has to be stressed is that this is potentially lethal for the horse so, if you believe they do indeed have the condition, it has to be dealt with and not ignored. You will know if this is happening due to a discharge coming from their nostrils, which is a result of them being unable to eat food, along with saliva and mucas and may occur due to the horse trying to clear the blockage itself. If you feel they have it then call your local vet immediately.
As you would expect, a horse is going to experience a lot of discomfort, as well as some pain, if they have this problem. A familiar pose for one suffering it is with their legs spread, their neck being extended more than normal but also with their head hanging down, there may also be a sign of sweating.
If you believe this is the problem then try feeling under the jaw and along the actual esophagus yourself as in the early stages there may be the possibility of being able to massage away the problem yourself however only do this if you are experienced. Most people do need to call the vet as they will be able to examine the horse for any additional problems since there is a possibility of them contracting pneumonia or even a wind problem. It is common for the vet to give them a series of antibiotics to help prevent any further complications.
The main cause of this problem is due to the horse eating without chewing properly and can be seen even in young horses. To prevent them from getting it, or lowering the chances, try making their pelleted feed wet as it will help them to swallow it. Aside from this, work on stopping them from bolting their food which means breaking the habit and a good method is by adding rocks to their grain so they need to slow down in order to source the actual food.
Even although pelleted food was just mentioned it has to also be pointed out hay can cause a problem. For this you need to make sure it is not dry just like before. Keeping with the food theme, offering them a full carrot or apple may also lead to them developing the problem but do not stop giving them it but do cut it up into smaller parts first.
Finally, with regards to things they have ingested, you need to look at foreign objects which means bits from a fence or anything else lying around. You may also find they ingest the twine from hay by accident so do check this before offering them it.
When you see that they do have signs of choke then get them in to somewhere dry and clean until the vet appears and avoid giving them anything to eat or drink. This will keep them calm and when the vet arrives they may do several things including passing a tube down in order to find the blockage, give a sedative, then try and flush it out. Surgery is only required in extreme situations.
After they have recovered your main concern then has to be on making sure this is not a recurring problem. Do look closely at the feed situation however consider making gradual changes as they will be more likely to accept it and always ensure the have a good source of nice fresh water. When it comes to feeding them do it on a regular basis, they are not designed to fast and like regularity, so they do not gulp it down when they do eat. Ensure their stable is clean, and does not have any foreign objects, and remember to cut up those treats before letting them have it. This final part may come down to teaching visitors about it as they will not know of the potential problems they may cause.
So if you even just fear they have this problem then call a vet as soon as possible. Doing so may save their life and will at least stop them from being in pain or discomfort for any length of time.