You arrive at a client's office on an international business trip. You're making small talk about your flight there. You say:
It took forever to go through customs.
"Forever" is a common exaggeration that means "a long time". One way to use this is when you're complaining about how long something is takes:
It took forever to convince him to come out with us.
"Customs" is a section of government that controls all of the things that come into a country and go out of it. When you fly to another country, there's usually a customs officer who checks your customs documents and asks you questions about what you've brought with you. This process is called "going through customs".
Note that you also have to go through immigration when you fly into a country. The immigration officer checks your passport and visa, and asks questions about why you're visiting the country, how long you'll stay, etc.
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