For a description of MariaDB 5.5 see the What is MariaDB 5.5? page.
MariaDB 5.5.61 is a maintenance release. It includes bugfixes and updates, including from MySQL 5.5.61.
A full list of all changes is in the changelog.
For a full list of contributors to MariaDB 5.5.61, see the MariaDB Foundation release announcement.