A harvester is someone who gathers crops when they are ready — the people who go into the fields to cut and collect rice, wheat, fruit, and so on. For a long time this was hard work done by hand, so the word can still mean that farm worker. Though actually, in modern English it is just as often the machine that does the same job: a big farm machine that cuts the crop and collects it. A combine harvester is the large one that cuts the grain, knocks the seeds out, and cleans them in one go. The word is ordinary farming and news English, not slang and not rude. People sometimes stretch it: a computer program that gathers lots of information may be called a data harvester, using the farm picture as a picture only. The core idea stays the same: the one who gathers the crop.