A journo is a journalist — someone who writes or reports the news. The word is casual and in-group. You hear it in Britain and Australia, in newsrooms and among reporters talking about their own job. Americans almost never say it; they say reporter or journalist. It is not a word for a school essay. The -o ending makes it sound like a nickname, short and a bit cheeky, though actually grown reporters use it of themselves with no baby-talk in it. Sometimes it is warm. Sometimes it is a little dry, as if the speaker is rolling their eyes at the trade. Either way, it still means a news person, not a diary and not a magazine.