Matrix messages when loading lme4

It means that the Matrix package includes functions with the names
listed, which as long as Matrix is loaded, supersede the original
versions of those functions in the base and stats packages.

If you look at one of the help pages, it should explain the change to
the function. Usually this is some additional functionality and the
function should be usable as before, if desired.

For example, ?xtabs (or ?Matrix::xtabs) gives this notice: "This is a
fully compatible extension of the standard stats package xtabs()
function with the added option to produce a sparse matrix result via
sparse = TRUE."

Hope this helps,
Dan