TRON is a debugging command in old dialects of the BASIC programming language.
It was used primarily for debugging line-numbered BASIC GOTO and GOSUB statements.
In text-mode environments such as the TRS-80 or DOS, it would print the current line number which was being executed, on-screen.
[1] TRON and TROFF were made obsolete by the advent of multitasking windowing environments, which permit a developer to switch back and forth between program output and source code.
Modern IDEs take advantage of this to allow far more sophisticated debugging (line-by-line step-through execution of source code, examination/modification of variables, etc.).