Solon has become something of a mythical figure so what exactly he did (as opposed to what people later claimed that he did) is not completely clear. However, scholars tend to agree that he instituted a series of reforms that saved Athenian democracy from breaking up over issues of class.
One major thing that Solon did was to make it so that Athenians would not end up as slaves. He did away with slavery for the payment of debts and he redistributed land so that more people could own land and make a living. In a related action, he made sure that more people had a say in government. He did this by breaking the citizenry up into 4 groups based on wealth. Each group had a certain set of offices it could hold or political duties it could engage in (like participating in trials). This made sure that all levels of society had a part in government.
By doing these things and others, Solon ensured that all classes would have a decent part in their society. This helped to save Athenian democracy.