Basically, the victory that Washington won at Trenton inspired people both in America and Europe to have greater confidence in Washington himself and in the potential for the rebellion to succeed. This had some important consequences.
In the colonies, people had been losing confidence in Washington. Many soldiers were going to be leaving the army after their enlistment terms were up at the end of the year. The victory inspired many to stay and convinced others that the cause was not lost.
A similar thing happened in Europe. There, the most important consequence was that the French started to take a more active role in supporting the Patriot cause because they believed that there was a serious hope of victory.
Please follow the link -- it has a good discussion of these changes.