The British considered the signatories to be traitors, thus punishable by death. If the Revolution had failed, the signers would have been subject to execution, property confiscation, and their families would have been ostracized.
Some of the signers, like Thomas Jefferson and Benjamin Franklin, were wealthy landowners, and they had much to lose in terms of property and prestige. Others, like John Adams, were lawyers or merchants who had more modest means, but they too had a lot at stake.
All of the signers knew that they were risking their lives and their livelihoods by signing the Declaration of Independence, but they were willing to do so because they believed that the cause of liberty was more important than their own personal safety.