In my experience it is a very very loose collection of ideologies that share the common idea that there is no innate hierarchy between people and that helping others does not necessarily erode your own place in society.

Edit: Oh, it was an ELI5, then a leftist is someone who thinks other people in their sandbox does not make the sandbox worse.

A leftist is someone who, in the French national assembly, sits to the left, as seen from the lectern: Supporters of the Ancien Régime (monarchists) sat to the right, revolutionaries (of every colour) to the left.

As such the core tenets of leftism are Liberté, Égalité, Fraternité, in short, enlightenment values later coalescing into humanist values, as well as the metric system. Which is why the US cannot into leftism.

For me, the defining characteristic of leftism is trying to achieve social equality through redistribution of wealth and power from those who have plenty to those who don't have enough.

Collectivization of industry, ie a rejection of Private Property. FOSS is leftist as it rejects individually owned IP and the profit motive.

Socialism, Anarchism, Communism, etc. are examples of leftist ideologies.

If you want a true ELI5, instead of one dude owning the factory and therefore everything the Workers create in it, imagine the Workers owning the factory and democratically deciding how to allocate profits and whether or not to elect a manager to help facilitate this.

A political catch-all for non-conservatives

Its so big and wide and vague, theres no good answer.

But basically, communists and hippies