The aports tree contains for every package/program a directory with the corresponding APKBUILD file. APKBUILD is a file containing "recipes" on how something should be built/compiled. These files are used when building alpine from source.
When Alpine Linux is compiled, you will no longer see (or have use of) the APKBUILD. It is not included in the 'iso' or 'tar.gz'. The abuild script reads the APKBUILD and executes the steps needed to create a package.
There are some directories placed in the aports tree. A short description of every directory can be found in this section.
main contains the basic set of packages for Alpine Linux.
All new packages goes to testing first. Package will be moved to main if there positive feedback or other good reasons.
The packages in non-free are violating the standards of the Free Software Foundation (FSF) about copying, redistributing, and modifying computer programs in one or another way.
unstable is the place for unstable packages. The packages in this directory often have several issues.
Fetch latest APKBUILD files
While inside your build environment you need to install some needed packages and you need to fetch the APKBUILD's from the server (fetch the aports tree).
Note: You only need to do these 2 steps once! Next time you can skip this part.
When the above is done you might be interested in fetching the latest updates.