To begin with a little bit of history. Back when liberalism was originally created, the western world was a class-based society. Status, inherited from one's parents determined one's wealth and place in society. Children of nobility enjoyed privileged existence at expense of everyone else. Actual privileges vary between countries but very often nobility owned all land in the nation, were tax except and even had special privileged laws and courts just for them. Police could arrest a commoner but not a nobleman, only fellow noblemen could judge another nobleman. The majority of common people, had to pay, rent to nobility, tax to the state and were treated as inferiors in law, in court and in social situations. Such an unfair and unjust society. As we are now it's hard to believe all that was the case mere 250 years ago and, in some places, even less than that.
Back then Liberalism aimed to fight against all that. Liberalism aimed to bring people equality and freedom, abolish unfair class system and special laws based on social status. It both succeeded and failed at the same time. Liberalism did succeed in changing the laws and abolishing any legal inequalities. Nowadays we have equality before law and nobility special privileges were relegated to history books (mostly).
On the other hand, liberalism created a new different kind of social stratification, based on wealth instead of legal status. Marx called them bourgeoisie and proletariat. Rich (bourgeoisie) had enough money and knowledge to create new businesses and shape the world as we know it. Poor (proletariat) did not have enough money or knowledge and thus were relegated to working for the rich.
All in all, liberalism failed to achieve its original objectives of creating equal and fair world for everyone. Rich and poor replaced lords and commons, but fundamentally society was just as divided into two tiers of people as it was before. Technically new division is not based on heredity, but in practice most wealth is inherited rather than created from scratch. Nobility too was once conditional to military service but gradually became just a set of inherited privileges. See the pattern here.
What liberalism initially meant to be, before it gradually fused with its original enemy and formed conservative liberalism or liberal conservatism. An alliance made in hell. Conservatism embraced certain aspects of liberalism to stay relevant and electable meanwhile liberalism lost its soul and became shell of its former self, touting token achievements while failing to notice glaring problems. So much so that Socialism came to supplant its place as champion of the people. However, Socialism has different rather illiberal ethos and far from a successor to the dreams and aspirations of freedom loving people.
Social Libertarianism is a resurrection of the original classical principles of liberalism. Social Libertarianism is neither Socialist nor it is Conservative in its principles. It's an adaptation of liberal principles to the realities of life as well as to the realities of 21st century socio-economic environment. An ideology to achieve real freedom, equality and justice. To that end Social Libertarianism embraces those policies and ideas that are conductive to achieving its objectives while opposing those that are contrary to its aims.
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