It's not a monolithic term, but there are commonalities that define it. For example, all hunter gatherers lead at least partly nomadic lifestyles, without any fully permanent settlements (though they can have constructions they gather to, and sometimes spend significant parts of a year at). As such, they won't have, or at least not rely significantly on, things like mills or cutlery. This will mean that, in all climates, hunter gatherers will have a diet that requires much more physical effort to tear off and chew than most agricultural societies, and definitely much more so than any modern diet.