Some businesses, like gas stations, restaurants, and hotels, put a "hold" on some of your available funds until your purchase is finalized and posted to your account. This is also known as an "authorization", which protects the business until the exact amount of your transaction is determined and posted. The Amount on Hold is the sum of these transactions which have not yet been posted.
Gas Stations: When you pay for fuel at the pump, the gas station puts a hold (usually $75 to $125) on your account. This means you cannot use that amount for anything else while the hold is in place, which could affect your ability to use your card for other purchases if your Available Balance is less than your requested purchase amount.
For example, if your Available Balance is $100, and you use your card to pay at the pump for gas, the gas station places a $75 hold on your account and your Available Balance is immediately reduced to $25 (even if you pump less than $75 worth of gas). If you then try to make another purchase for more than $25, it will be declined because your Available Balance is only $25 at that time. Your Available Balance will not increase again until the hold is released or you add funds to your account.
A gas station hold is typically removed within a couple of days, after the actual amount of your gas purchase is processed and posted to your account.
Restaurants will typically place a hold (authorization) on your account for the amount of your bill, plus an anticipated tip. When the final transaction is posted, the hold is released from your account. The final amount posted could be more or less than the amount held, depending on how much you tip.
Hotels and Rental Car Companies are other examples of businesses that may put a hold on your account when you use your card to do things like make a room reservation or rent a vehicle. The hold amount varies depending on the company's estimate of how much your final charges may be. The hold is released after your final bill is determined and posted to your account, which can be a few days after you complete your business with that company.