L stands for luxury, it's not an indicative of quality as such but that being said…they are generally better.
For example the 16-35L is better than the 18-55mm. Sharpness aside, the 16-35 offers weather sealing, faster aperture, better build quality, faster focusing and nicer bokeh (more aperture blades).
Those are the elements that usually apply to an L – build quality, weather sealing, aperture, focusing and of course, sharpness too.
There are some exceptions. The 17-55/2.8 is considered to be a L standard lens in terms of sharpness, although I don't think it is in the same ballpark in build quality as a 24-70.
The 45 TSE is not an L but it comes with a lens bag and a lens hood which are normally reserved for an L. Also it is all metal construction, there is more metal on it than my 35L, it is also stupidly sharp.