Slant is the angle letters make with the baseline. To measure it, look at the tallest strokes and imagine a line running through them.
A right slant often belongs to writers who show feeling easily and lean into people and ideas. A vertical slant tends to belong to writers who prefer to think first and react later. A left slant often belongs to writers who protect their inner world.
None of these is better than the others. They are simply different ways of handling emotion.