Tor, short for The Onion Router, is a free network that anonymises internet traffic by bouncing it through a series of volunteer-run relays, each of which peels away one layer of encryption — hence "onion". The result is that no single relay knows both who you are and what you are accessing, which makes it hard to trace activity back to a user's real IP address.
For OSINT, Tor matters in two ways. It is the gateway to the dark web — sites with .onion addresses reachable only through the network — and it is a tool investigators use to separate their real identity from their research. Used carelessly it offers false comfort: browser fingerprinting, logins, and downloaded files can still deanonymise a user. Strong operational security around Tor matters as much as the network itself.