If we're holding an object as security for a loan, it must be returned to the borrower (who is the item's owner) on as as-needed basis. (Since this is a negative mitzvah, I guess it should be phrased "we're not allowed to hold onto the object when it is needed.") The archetypical example is, if we are holding someone's blankets as collateral, they must be returned to him at night. If not, how will the borrower be able to sleep?
The reason for this mitzvah is that God likes it when we're good to one another. He doesn't like it when we act like jerks.
This mitzvah applies to both men and women in all times and places. It is discussed in the Talmud in tractate Baba Metzia (113a-b) and codified in the Mishneh Torah in the third chapter of Hilchos Malveh v’Loveh. This mitzvah is #240 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #61 of the 194 negative mitzvos that can be observed today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.