What is the meaning of the introduction of the Declaration of Independence?
There is no part of the Declaration of Independence that is officially called the introduction. However, when people talk about that document, they often use the term “introduction” to refer to the following part of the text:
When in the Course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation.
I will base my answer on this portion of the text.
Basically, this introduction is meant to say what the text as a whole is about. It says that, when it becomes necessary for people in one part of a country to split off from that country (that’s what “to dissolve the political bands which have connected them with another…” means), the people who are splitting off need to tell the world why they are doing so.
The introduction also implies some things about what will be said later on in the Declaration. It implies that there are natural laws (or divine laws) that entitle countries to be free and independent. It implies that there are times when it is right, and indeed necessary, for people to break away from their government.
Thus, the introduction tells us what the overall purpose of the document is and it hints at some of the important points that will be made in the rest of the text.