It stands for tom/ next repo (tomorrow/ next repo); a type of forward repo that is executed on T+0, with the opening leg of the trade settling on T+1 (tomorrow), and the closing leg of the trade settling on T+2 (next). It is an overnight repo (though it differs a little in terms of forward time horizon) in which a one-day repo rate is applicable to trades starting next business day.

Tom/ Next repos can be traded on a T+1 / T+2, T+1 / T+x basis (there are no restrictions on the return leg as the trades are individually managed).