A checksum is a number generated by adding up all the data in the file you want to check. If any characters change, the checksum will differ from the original and an error message will be displayed so you are able to investigate the changes. The files directory can also contain other files that are required by the port but do not belong elsewhere in the directory structure. A patches directory. This directory contains patches to make the program compile and install on your FreeBSD system.
Patches are basically small files that specify changes to particular files.
A pkg directory. This directory normally contains three files. Occasionally, there will be more than three, but it depends on the port. Most only require three. This is a one-line description of the program.