It's called kerning, which is an additional spacing applied to specific pairs of characters1.
The aim is to have a perceived equal spacing between glyphs. Mathematically equal spacing based on the bounding box of each glyph doesn't always work since glyphs have very different shapes; some having a lot of empty space within the bounding box, and some hardly any. Glyphs that commonly need kerning are characters with slanted forms (e.g. "W", "V", "A") and characters with "arms" or large crossbars (e.g. "T", "F").
Take the string "AVAIL" as an example. With no kerning applied, the empty space between the "A" and the "V" is very obvious:
So we apply some negative kerning to bring it in:
Another thing to look out for is collisions. Take the following example... without any specific treatment, the hood of the "f" and the dot of the "i" collide, so we need to pull them apart:
Collisions with "f"s are commonly fixed with specific ligatures too (but I digress...)
Watch out for collision between serifs too. The "y" and "p" here looks sloppy, for example:
See also: What are some common kerning pairs?
A few common techniques
Flipping the text lets you focus on the shapes and space without being distracted by the actual letterforms, words and meaning; you can just treat them as shapes...
Squinting or crossing your eyes to blur your vision of the type gives you a good view of the overall contrast and use of space (This is a good technique not only for kerning but any layout or spacing). You can set up an actual blur on your type in whatever software you're using too if it helps...
Glyphs — which I've been using for type design recently — has both of these as features built in to the editor:
A more abstract technique I've seen mentioned a few times is the balloon technique... You essentially just need to imagine a ballon between each character. The balloons need to fit between the characters with as little space between the balloon and the characters as possible and without spilling above or below the x-height; all while maintaining the same area.
As with everything, the best way to learn and improve is practice, practice, and more practice.
The Kerning Game is a nice little tool to practice your kerning and compare it against an experts kerning...
1 Or sometimes more. See What is a polynomial kerning table?