There are two parts to this statement that bear discussion.
First, they say that this idea is "self-evident." This means that it is obviously true. So what they are saying there is that it is obvious to everyone that all men are created equal.
Second, we have to think about what they meant by all men being created equal. To begin with, we must note that they clearly meant to say "men." There was no idea among most people in those days that women could possibly be seen as equal to men. We must also note that they clearly did not envision black men being created equal. Finally, we need to ask what they meant by "equal." Clearly, they did not mean that they believed that all men were born with equal intelligence or equal ability to do various things. What they did mean is that all white men were equal in the sense that they all had the same fundamental rights under the law. All men, they were saying, had the right to their life, liberty and their pursuit of happiness. They were not saying that all men had the right to achieve happiness, only that they all had the same right to pursue it. They were also saying that family status did not matter. A person born to a rich family was not automatically better than someone born to a poor family. This was an attack on the system of aristocracy and monarchy.
So, we can generally say (because we do not know what it meant to each signer of the Declaration) that this phrase meant to the founders that it was obvious that all white men were born with a certain set of natural rights and that no white man was, at the moment of his birth, better than any other.