hi @gylooo , welcome to the community.

1. You use a T-Mobile Netherlands simcard in your phone?
2. Did you use this simcard/phone combination already in the USA?
3. If this is the first time you have the sim in the Iphone and it reports this sim card restriction, you need to contact the vendor of the Iphone. They locked the phone for usage with another provider. In the meantime you can use your T-mobile Netherlands simcard in a different phone, so you have mobile services.
4. If you bought the phone in the USA from T-Mobile USA; T-Mobile Netherlands and T-Mobile USA are two total different companies. Only the brand name show some simularity. 

Hallo

I didn't use a Sim phone card in the United States. I bought my phone in the mall the day I left, and when I returned home, I found out it needed activation. I don't quite understand what it means, so I can only use Wi Fi to access the internet now.

Can you help me ？

Ok, so if the phone reports that it has SIM restrictions, it means it does now allow the T-Mobile Netherlands sim to be used. It is often used when carriers sell sponsored phones in the USA. 

“ Contact your network provider

1. Only your network provider can unlock your iPhone. Contact them and request an unlock. After you've submitted the request, it may take a few days for the process to be completed. Contact your network provider to check the status of your request.” 

Whatthey mean here is that you need to contact the Network provider in the USA of which you bought the phone in the USA to have them send a release code of the simlock. That might charge you for that request.

what is pin

lcannot

There can be 2 pincodes,  a pincode of your phone and a pincode of the simcard. The pin on your phone is created by yourself. The original pincode of the sim can be found on the larger plastic card where you toon the smaller simcard from. Or the original pincode can be found in your my-tmobile app or website. But normallly you would choose your own pincode as soon as possible.

The simlock pin is set by the Operator that sold the phone. I guess that that  is T-mobile US?

That is a different company. Contact them to get the simlock key for your phone. T-mobile Netherlands can not help you.