It's helpful to think of it with a historical perspective. Liberals emerged in opposition to feudalism, mercantilism, and monarchism. They believed that all men are created equal and thus should have equality before the law. Life, liberty, property etc. This obviously went hand in hand with capitalism since it meant anyone (in theory) could own property, and not just aristocrats, and become rich through their own "hard work" without government interference. Liberals believe in constitutional limited representative democracies and capitalism.
In the US, liberalism split into "liberalism" and "conservatism", with "liberalism" being a proactive welfare state and "conservatism" being laissez-faire. This has further changed to the point where conservatives are starting to openly embrace fascism.
"Leftism" is a broad term covering a variety of anticapitalist movements, but its core principles relate to egalitarianism, destroying hierarchies, radical change, etc. Socialism developed out of the struggle between owners and workers within capitalism. Leftists want to upend capitalism and all other systems of oppression in general.
In essence, liberals were once revolutionary but are now the status quo, with the exception of the fascist wave sweeping the world. Leftists are people who want to go further beyond liberalism because they see it as insufficient to create a truly egalitarian world.