I think it's usually just the components of a Lorentz transformation, but you shouldn't assume that every author uses that convention.
I'm not a fan of the [itex](\Lambda^\mu{}_\nu)[/itex] notation. I assume it denotes the matrix with [itex]\Lambda^\mu{}_\nu[/itex] on row [itex]\mu[/itex], column [itex]\nu[/itex]? I would just write [itex]\Lambda[/itex].
Note that in most books, the horizontal positioning of the indices is important.
