You don't know what kerning is. You can make out that you are on a typography site so you have a basic idea of the kind of things it could be referring to. You notice that 'keming' and 'kerning' are visually really close, and that keming is defined as a wrong type of kerning. The only difference between the two words is 'rn' and 'm'. Thinking about what typographers would find interesting about this, you realize that sometimes if letters aren't separated properly, it would make it hard to read some words unambiguously. This could be an important enough issue for typographers that they might have a specific term for it. So maybe kerning describes the idea that letters should be properly spaced inside words, or maybe only the spacing of particularly troublesome letters, or even just the 'rn' combination. Either way you have a plausible idea what kerning means. Now you look at whether the joke makes sense with this idea. You see that it does, that keming would be both literally and visually wrong kerning, and the definition makes for a particularly clever pun. So everything fits together, and you can be reasonably confident that you get the joke and the broad meaning of kerning. You just reverse engineered the joke. Thats hacking.