From their site <https://www.qt.io/download-open-source>: LGPL – Any modification to a Qt component covered by the GNU Lesser General Public License must be contributed back to the community. This is the primary open source Qt license, which covers the majority of Qt modules.
If you're not familiar, in practice, the difference between GPL and LGPL is that LGPL does allow dynamically linking a library with a closed source program, without requiring that the program also becomes GPL/LGPL. This is compatible with most development as long as you either don't modify Qt, or are ok with those modifications becoming open source.