“Tachiyomi” literally means “standing reading” and describes the practice of going to a bookshop and standing there reading the magazines or books.
In my native NZ, within 5 minutes of me entering a bookshop and opening up a magazine, a not so friendly shop assistant would inevitably pipe up with “are you going to buy that?”. In Japan however, there is a lot more acceptance of the practice – especially for magazines. Less so for manga though – you’ll usually find these sealed in plastic to prevent cheapos from reading the whole thing at the bookshop.
Some places might have a sign like ‘立ち読みをご遠慮ください。’ (please refrain from standing and reading) but this is very rarely enforced.