TES stands for The Times Educational Supplement — a British publication for teachers and everyone who works in schools. It began as a weekly supplement to The Times newspaper in 1910, later became its own paper, and is now mainly a website. What it carries is school news, teaching ideas, opinion pieces about education policy, shared lesson materials, and — the part teachers care about most — job advertisements. For decades in Britain, 'looking in the TES' was simply how a teacher found a new post; if a British teacher tells you they saw a job in the TES, that is what they mean. The organisation now calls itself just Tes (often written that way, with only the T capitalised), having dropped the full old name, so you may see both forms. You say the three letters separately: T-E-S. Note that these letters stand for other things in other fields — but this entry is the teachers' paper.