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how 1 mm of hg becomes 1 torr? (please give wider explanation.)

A millimeter of mercury is a manometric unit of pressure, formerly defined as the extra pressure generated by a column of mercury one millimetre  high. It is now defined precisely as 13.5951 × 9.80665 = 133.322387415 pascals.It is denoted by the symbol "mmHg".

Even though it is not an SI unit, the millimeter of mercury is still routinely used in many fields, including medicine and in applications of vaccum technology.

One millimeter of mercury is approximately 1 Torr, that is, 1/760 of standard atmospheric pressure. Even though the two units are not exactly equal, the difference (less than 0.000015%) is negligible for almost all practical uses.