It's just a directory full of Makefiles that fetch the source, build a package and install it. Here is the OpenBSD documentation on what the ports tree is. Basically, if you want to install chromium, for example:
And you'll have built and installed a package from source. The directories are laid out like /usr/ports/$CATEGORY/$PACKAGE: for example, irssi is in net/irssi, dmenu is in x11/dmenu, terminus font is in fonts/terminus-font and so on. Generally you might want a ports tree if the prebuilt packages weren't built with a specific feature you want (support for a scripting language, a specific sound server etc) or if there are too many features and you want to cut down.