When you install software on Mac (or PC) normally, you just drag & drop your software or run a specific installer.
When you install software in UNIX, quite often this software uses another software that uses another software that uses another software... and each software needs to setup different variables... and each software exists in several more or less compatible versions... fink accesses databases of software descriptions to validate all relations between all pieces of software necessary for the software you want to install and does all the necessary installations and setup for you.