When funds are held for any reason, such as a preapproval for a debit card purchase, you will see a difference between your actual and available balance. Below is an example of the Account Summary section which lists your current activity such as holds or pending ACH.
When you use your debit card, the merchant may place an authorization hold to confirm you have sufficient funds for a transaction or for a deposit for a potential future transaction. While this hold is in place, the funds are generally not available for withdrawal.
Clicking on the "Current Activity" icon in the Account Summary section, will produce information regarding the specific item(s) being held.