If you bought something from China stores in Aliexpress or other sites, the seller may use epacket to ship your package, this is a quite fast shipping service, and the shipping price is much cheaper compare to DHL/UPS and other international express companies.
What does "held by customs" means?
Sometimes the tracking info will show "held by customs", it just means the item is being checked in customs. About a few days letter, it will update with "customs clearance".
But the key point is, if your tracking number is a epacket number(starts with "L" and ends with "CN"), and the tracking info shows "held by customs" in China, this is probably a bad signal. It is maybe a returned package.
For example, LX224826860CN is a epacket tracking number, the tracking info of this package is below:
Destination Country - Not Found(Modify)
Origin Country - China
2017-01-1309:59:51 Delivery, GuangDongShengShenZhenShi 51802168
2017-01-1309:39:53 Delivery arranged, GuangDongShengShenZhenShi 51802168
2017-01-0614:36:08 Held by Customs, SHENZHEN
2017-01-0516:47:00 Despatch from Sorting Center, GuangDongShengGuangZhouShi 51000061
2016-12-0218:54:00 Posting, GuangDongShengShenZhenShi 51802168
This is seems to be a delivered package, but actually, the real destination of this epacket item is Turkey, so this is a returned package. It shows "Held by Customs, SHENZHEN", and did not pass the customs. So, be careful if your epacket tracking number shows: "held by customs", you can contact the seller and ask them to resend a package to you.