I asked about my SN ... I checked it on the sait of apple and I found that the SN of my phone is replaced... what that mean please ??

It means that your iPhone (current serial number) has been returned to Apple (most likely for a repair job) and apple had issued a replacement iPhone (with a new serial number with part number or model number starting with letter "N"). Apple will stop supporting this replaced iphone with old serial number, usually the returned iPhone will be repaired or have the broken part replaced and will be put back to circulation with a new serial number and a part number or model number. We will never know if the iPhone will be sold as new iPhone or as a replacement iPhone. But I personally think Apple will not sell it as new iPhone, most likely be sold as refurbished iPhone.

If you bought your iPhone from a retailer, maybe Apple forgot to change the serial number, but if you bought it from other people, maybe it is a result of a crime?? I don't know for sure, just assuming.

If you try to search for the replaced serial number, apple will respond with this:

It's very bad. It should not be in circulation at all; it is a stolen phone. Either Apple sent the owner a replacement, expecting the user to return the old one (and he instead sold it to you), or it was stolen from a shipment back to Apple.

Apple won't support it, and it may be blacklisted.

Presumably you had an incident requiring your iPhone to be replaced. Perhaps that is what happened.

Without more information, it's not possible to know for sure.

Where did you get the phone? That message means that the phone was returned to Apple for repair. If it wasn't new to you then most likely it was stolen in transit to Apple.