What is a summary of the Declaration of Independence?

The Declaration of Independence is one of the most important documents in American history. It set out, for the first time, the reasons why the American colonies should break away from Britain and declare their independence.
The Declaration starts off with what's called a preamble. This is kind of like a mission statement, a way of letting everybody know what the signatories to the document are doing and why. It's a statement of intent and a justification for what's about to follow.
Then we move on the first section proper. Here, Jefferson famously justifies independence on broad philosophical grounds:

We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.–That to secure these rights, Governments are instituted among Men, deriving their just powers from the consent of the governed, –That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their Safety and Happiness.

We are all endowed by God with natural rights. We also have the right to remove any government that doesn't respect those rights and replace it with one that does.
There then follows a long list of grievances against King George III. Among other things, he has persistently undermined attempts by the American colonists to set up legislative institutions. He has also levied taxes without the colonists' consent. Even worse, he has undermined the nation's security by, among other outrageous acts, confiscating American ships and hiring foreign mercenaries.
Finally, the American colonists have made repeated attempts to reach an amicable settlement with the British, but they have all been ignored and treated with contempt. There's nothing for it. We have no alternative but to declare our independence from Britain once and for all. In doing so, we have established a new nation. That nation is to be called the United States of America.
https://billofrightsinstitute.org/founding-documents/declaration-of-independence/