What does Lxi means in a car?

Well, it depends on which model you are talking about, I'm searching for a similar but slightly more specific answer.

These are "add on" feature/trim packages. I have a Sebring Convertible LXi. "Sebring" is the base model. LX would be a package with additional features, and LXi would have even more features. I'm searching for the exact features of each package myself, but an example would be:

LX: power Windows/locks, upgraded radio, power seats, 4 cyl engine

LXi: 6-way power seats, leather, power mirrors, black and chrome interior/exterior trim, 6 cyl engine

Sometimes packages like these can include additions under the hood, like the ram-air package on the grand am, or adding a turbo, etc, and others include exterior features, like different wheels or body accents along the fenders or rocker panels, vents, etc.