Who they are
What do we mean by ”uncontacted tribes” ?
Tribal peoples who avoid contact with outsiders. These could be entire peoples or smaller groups of already contacted tribes.
Does that mean they have no contact with anyone else at all?
No. Everyone has neighbours, even when they’re some distance away, and they’ll know who they are. If it’s another tribe, perhaps also uncontacted, they may or may not have friendly relations with them.
Have they ever been in contact?
In some cases, probably. Some may have been in touch with the colonist society in the past, and then retreated from the violence which that brought. Some may once have been part of larger tribal groups, and split off and moved away, fleeing contact.
So they aren’t necessarily living as they were in past centuries?
Nobody is. Some Amazonian groups even had guns, from inter-tribal trading, before they’d ever met a non-Indian. Most uncontacted tribes have used some metal tools, which they have found, stolen or traded with their neighbours, for many years or even generations. Uncontacted peoples in India’s Andaman Islands use bits of metal from old shipwrecks, for example.