The fretboard is the main way of editing a step on the tab. The fretboard is a visual representation of a fretted instrument's fretboard where the spaces are clickable so you can set which frets on which strings should be held/played. Clicking spaces on the fretboard will edit the currently selected step.
To set a string to play on a certain fret, simply click the fret on the desired string. Clicking it again will unset it so that the string no longer plays. If you click a different fret on a string that is already set to a certain fret, it will switch it to be playing the fret you clicked. You can set a string to play open/fret "zero" by clicking all the way to the left edge of the first fret.
Right clicking a fret on a string will "star" it, which puts an orange marking on it. Right clicking a starred fret will remove the star. This has no persistence in the save file or function in QuickTabs but it is useful for tab writing (for example: if you are writing out a fingerpicked chord on the tab in a certain picking pattern, you can star all the frets in that chord for easy reference.)
The buttons in the column furthest to the right of everything on the Fretboard allows you to set the sustain time (how long the strings you are currently editing should continue playing for.) It uses musical notation icons to represent the different note lengths. The currently set one will be highlighted. Clicking any note length button will change the note length for the current step.
Between the fret area and the note length buttons are a pair of left and right buttons. These can be used to scroll the fretboard in case you need to access a high fret that is not normally on your screen.
The fretboard and its frets can be made larger from View->Large fretboard. You can also toggle visibility of the fret counter at the bottom with View->Fret counter, as well as the navigation dots on the fret area with View->Fret navigation dots.