Kerning refers to the process of adjusting the space between letters in typesetting and graphic design.
The word originally comes from the French carne, meaning projecting angle or quill of a pen. In the days of manual typesetting, letters were placed on individual metal blocks known as glyphs. If a letter overlapped the following letter (e.g. a capital T before a capital A), the overlapping parts (the bars of the T), would protrude over the edge of the glyph, and these exposed parts of the letter were known as kerns.
You might not think kerning is important, especially because most digital text is automatically kerned for your pleasure. Here’s a nice example from Wikipedia of the benefits of kerning:
See how bringing the A and V together so they overlap makes it look much better? To contrast, look at the following sign:
See how the AVA fit together well, but this sets them apart from the rest of the name? This could use some kerning to bring the other letters closer together.
You’ll probably never need to use the word kerning, but it’s nice to know a little bit about what goes on behind the scenes of writing, the subtle magic that happens to help us read smoothly, and keep the world turning. And if you’re ever designing a sign, now you might be a little more conscious of your kerning, lest you end up like the poor people involved in these examples of bad kerning.