Installing applications (which roughly equates to "installing packages") is a fundamental part of the Linux experience. A Linux distribution, such as Salix (or Slackware, on which it is based) could be said to consist of its repositories, where the software available for that distribution is kept. These packages have been compiled with the kernel and build tools provided by the distribution and tested to ensure they work together. Salix, like most distros (but not Slackware) provides full dependency management, which means that any items the package needs to run are installed too -- and also that they are guaranteed to be available.
Installing extra software is also possible through flatpaks and
Flathub. Just navigate to the software you wish to install and press
the "Install" button on the webpage. If your browser defaults to
downloading the .flatpakref file instead of opening it,
just open the Downloads folder and double-click it inside your file
manager.
You can also create and install your own packages, for which Salix
provides a suite of console scripts, especially slkbuild;
for further information, see the wiki pages on packaging. When
installing packages from any other source, proceed with caution.
slackbuilds.org (SBo) provides build scripts for further software not present in the repositories. This means the package will be built on your system before being installed. Salix provides a command-lie tool, slapt-src, for this purpose. But it has limited dependency management, as not dependencies are listed at SBo. Therefore the build is not guaranteed to be successful and some manual tinkering may be needed. Unlike the software provided in the repositories, these scripts are not maintained by either Salix or Slackware but by individual users, and as such are not the responsibility of either distribution, although help may be found on the forum. If the build fails, the last ten lines or so of the output usually provide an indication of the reason. Most commonly, this is a missing dependency, which may be a build dependency (needed to compile the package, but not to run it). Common packages that are required by several SlackBuilds, but not included as part of a standard Salix installation are cmake and linuxdoc-tools, so first make sure you have these installed if you encounter any problems. If you don't mind using a bit of extra space on your hard drive, then most of these problems could disappear by installing the slackware/d and slackware/l package sets:
sudo slapt-get --install-set slackware/d --install-set slackware/l
However, you shouldn't really need to revert to SBo often, if at all. The Salix repositories offer thousands of packages and even more are available through Flathub.