The Big Lebowski - What is it about?

First)  The Big Lebowski conceit - What if there was a world in which no one was they thought they were except for one man.

And then what if this one man tried to be something they weren't?

Now all of the characters are "out of place"  Every single one is a fish out of water.

Take the scene when Walter thrashes the sports car with a crowbar and says "Do you see what happens?  Do you see what happens when you fuck a stranger in the ass?"

This scene is similar to the "pee on your rug Lebowski."  "I'm not Lebowski, I'm the "Dude", man."

The Big Lebowski is about mistaken identity and the innocent bystanders who reap the wrath of it.

You could also say it's about "Identity Crisis"

The "dude" is comfortable with his identity.  He's a pacifist.  He's not trying to be something else when the story starts.  He doesn't need to change.

While Walter is pretending to be a Jew.  He's got an identity crisis from his ex-wife and his relationship.

When the "Dude" becomes an aggressor, a whole short storm takes place.

The "Dude" abides.  While all the other characters want money, or to win, or a child, etc.

"This usage of "dude" was still in use in the 1950s in America, as a word for a tourist – of either gender – who attempts to dress like the local culture but fails."  -wikipedia

The "Dude" is a fish out of water so to speak.  And when the case is solved he goes back to his water.

This movie is about a character who is perfectly in sync who then becomes outrageously out of sync and finally back to sync.

It's a circular narrative, dude.