Edicts

The Bakufu (the military government of Japan) were at fear of losing power so in order to ensure they keep their power, they issued a series of edicts.
They wanted to make sure foriegners didn't influence Japanese citizens more than they already have and tighten control over the daimyo.
They called it the National Seclution Policy and told the citizens it was a way to centralize the government and strengthen authority.

For a little a bit over 6 years the National Seclusion Policy was devoloped to control the interaction of foreigners and Japanese citizens.
This is a summarization of the actual Policy:

No Japanese ships are allowed to go to foreign countries.
Any Japanese people returning to Japan after living in a foreign country will be executed.
If you try to go out of Japan territiory you will be put to death.
If any Westerners come to Japan and start to spread Christianity, they will be thrown in prison.
No city is allowed to perchase merchandise brought by foreign ships.
No Portugueses ships are allowed to come to any Japanese ports. If this order in not obeyed then their ship will be desstroyed and the people on it will be executed.