GNU LGPL is a copyleft license.The difference between GNU GPL and GNU LGPL has to do with linking free software with a non-free software. The issue of transforming a free program to a non-free one is something else, which copyleft licenses deal with.
Read https://www.gnu.org/ for more info. For example, you can read https://www.gnu.org/licenses/why-not-lgpl.html.
