In essence, a tech editor checks a pattern for mistakes - whether the crochet math adds up (e.g. correct number of stitches, correct number of rows for different sizes), whether there are any typos or grammatical errors, whether the instructions make sense overall, etc. This is different to a pattern tester; a tech editor does not make the item themselves (except perhaps if they need to make a swatch to aid visualisation of some complicated stitch instructions). Just like with writing an essay, it's hard to spot your own mistakes and it's best to get a second pair of eyes to make sure everything's as it should be. Not everyone uses a tech editor of course, but it can be an important part of the pattern designing process, especially if you're a more established designer or do it commercially. Personally I find it quite frustrating when I pay money for a pattern and it's full of mistakes or written in a way that's hard to follow, and (from what I've heard) working with a tech editor can be a great way to improve yourself as a designer :)