If the package is on USPS, then there will be a tracking message that says the package has been seized by law enforcement. With other carriers, it is more likely that you will receive a message requesting that you contact the carrier. In that conversation, they will explain the situation in more detail.
When a package clears export customs, it is released by the country of origin for shipment.
Ultimately, this means that the first stage of the journey is underway, and there will be plenty of additional steps before you receive your package. But, a package that clears customs once is likely to clear it again.
In short, the message means that you will not be receiving your package on time. UPS uses this message to indicate that the original shipping plan has been disrupted, and the company cannot provide a new delivery date until the problem is resolved. There is no telling how long this might take without more information.
Forever 21 packages can and have come early to any number of customers. That said, it is not a standardized or normal occurrence, and reports of early delivery can largely be chalked up to good luck. A few factors can make it more likely that your package will come early, but ultimately, it’s uncommon.