The table in the upper-left contains the list of "Available" feats. Next to the word "Available" is a selection box that lets you pick which "view" you want. It is this view which will be used to determine the structure of the table below it. The same is true for the "Selected" feats which list the feats your character has acquired. By default the feats your character has selected as optional feats are in black text in the Selected table. Feats you have gained automatically (due to a race or class property) are displayed in dark-yellow text. Feats which you have gained virtually (typically this means there's some restriction on them) are displayed in magenta text. You cannot remove or change the selections of virtual or automatic feats. By clicking on the Filter icons in the toolbar (the icons with the magnifying glasses) you can filter which feats are included in the table. You can add a feat by double-clicking on it, right-clicking on it or clicking on it and the on the right-arrow button. Similarly you can remove feats, or alter the selections of optional feats which have choices, by double-clicking on it in the "Selected" table, or by right-clicking on it there, or by selecting it and then clicking on the left-arrow button above the table. As elsewhere in PCGen anything you don't qualify for is in red italics.
The info-pane in the bottom-left will give you details about any feat you click on - including any pre-requisites and its source.
The bottom-right section lists all your known weapon proficiencies. Since weapon-proficiencies are generally tied to feats they seemed to fit here best. If you have any optional weapon proficiencies due to you from your race or class selection, you can select them by clicking on the Choose button.
There are "sliding dividers" between the feat tables - you can click on this divider and slide it back and forth to your preferred location. There's also a divider between the upper-tables and the sections below which you can drag up and down to change the viewing area of each. There's another divider between the lower sections. This combination of 3 sliding dividers is common to most of the character-related tabs to allow you to change how much screen real-estate is devoted to each section. On most tabs there's a button which has 2 circling arrows that will reverse the orientation of the views between horizontal and vertical. This gives you many options on the layout of the tab.