The "bond" rather depends on where you end up. In principle it is the cost of your training course. You pay it, they provide the training. However the reason it is a "bond" and what actually happens is rather more complicated.
The nature of the training programme and the placement with a partner airline allows for the "bond" portion of the training costs to be assumed by the airline. This is tax advantageous from an input cost standpoint for the airline, and it has also has potential tax advantages for you.
Depending on the placement and whether or not the "bond" is transferred (purchased) by the customer airline, it might be repaid to you by way of monthly payments over a fixed period (5-7 years typically). These repayments (again depending on the airline) may be in addition to a cadet salary, or as part of that cadet salary. The level of that cadet salary likely reflecting the method of "repayment." The advantage to you is that a part of that repayment is not subject to income tax, but is subject to a smaller element of withholding tax. In any event it is advantageous to your net (bottom line) salary payment.
Some companies do not assume the "bond" and similarly if you are not placed for whatever reason or you obtain your own employment then the "bond" is simply passed across from the holding company to the training school.
In a nutshell that is how it works. The easiest viewpoint is that you are paying for training. Wave goodbye to the money in the same way you would paying for any training. Depending on where you end up, that money might be fed back to you but usually as part of a monthly salary payment. In that case you will gain a tax advantage (after the fact) such that part of your training costs were relieved from a tax viewpoint.
It is better not to get the idea that somebody is going to present you with a cheque for the "bond" portion of your costs and say "here is your money back." That isn't going to happen. However there are advantages to you, but they are spread over a fairly long period. Those advantages also do not apply with every partner. Those advantages are also subject to any future changes in inland revenue rules.
Again, it is important to understand that the repayment (if there is one) is completely dependent on the customer airline. Put simply, it is no longer your money, it is either the money that APL will use to pay your training costs, or it becomes the airlines money to keep or repay in whatever manner they see fit. This may go some way to explain why it is unclear.
If you want any further information I would need to obtain it from one of our cadets, but if you have a specific airline in mind you should be able to obtain it here, (after filtering the usual detritus!)