I assume that you are asking about the first paragraph of the first pamphlet from the series that we now call The American Crisis. In this paragraph, Paine justifies the right of rebellion by saying that the British government is acting tyrannically to the point that it is enslaving the American people.
In this first paragraph, Paine asserts that the British government is establishing a tyranny over the American colonies. He says that the British have claimed that they have the right to tax the Americans in any and all situations. He says that the British have claimed the right “to Bind us” in all situations. That means that Britain has claimed the right to force Americans to do whatever Britain wants. This, he says, is tyrannical. In fact, he goes as far to say that this has made slaves of the Americans. He says that, if this “is not slavery, then is there not such a thing as slavery upon earth.”
Paine is implying here that he agrees with Thomas Jefferson’s words in the Declaration of Independence. Jefferson said that people have the right to rebel if their government fails to protect their rights to “life, liberty, and the pursuit of happiness.” Paine is saying that the British government is actively trying to take away at least the last two of these rights. Therefore, he is saying, the colonies have the right to rebel.