There are numerous online politics quizzes that will put you onto a graph that looks something like this
This chart is from politicalcompass.org, and you can visit that site for more information.
This is what I assume you're talking about.
Left and "Libertarian" are on different axes on the chart.
Left is another word for "Liberal", and the "Liberal" party in the United States is the Democratic party. Usually, the Left is in favor of changing society into something new.
Right is another word for "Conservative", and the "Conservative" party in the United States is the Republican party. Usually, the Right is in favor of keeping society as it is, or bringing it back to what it use to be.
Libertarian means you value freedom, and reduced government control there is a Libertarian party in the United States, but it isn't as large as the 2 major parties.
Authoritarian is the opposite of Libertarian. Authoritarians are in favor of a lot of government control. I don't know what the Authoritarian party in the United States is, but I wouldn't be too surprised if one existed.