An old-school black and white film from Kodak.
It's been around for like 40 years, and has a distinct look when developed in certain developers that people like. It's usually shot around 400 ASA, and though it is grainy by today's standards, it was low-grain back in the day. These days people find the grain present in Tri-X shots is pleasant, and gives photographs a certain look they want in black and white photography aside from the lack of color.
The Ilford brand counterpart is probably HP5+.
Also, Tri-X professional in sheet and roll-film versions is slightly different, and is rated at 320 ASA.