Package Manager is software that allows a user in case of installation of new software, up-gradation of system, or updating any specific software and such sorts of things. In the case of Linux-based systems wherein one software has lots of dependencies whose presence is required for a complete installation of that software, applications like package manager become a much-needed tool on every system.
Each Linux distro ships with its default package manager to handle the above-stated functionalities, but of all these,
apt are most prevalent on RHEL, Fedora, and Debian systems.
If you’re looking for APT tool to block or disable certain specific package updates, then you should read this article.