We said in the previous mitzvah that if we're holding an object as collateral, it may not be withheld from its owner when he needs it. If we're holding his blankets, we must give them back at night so he can sleep; if we're holding on to the tools of his trade, we must return them by day so he can work.
We said in Mitzvah #585 that we may not just march into the house of someone who owes us money and seize property as security for the loan. Our current mitzvah also requires one who violated Mitzvah #585 to rectify matters by returning the object to its rightful owner.
The obvious question is: if I have to give the borrower his collateral back every time he needs it, what's the point? The answer is that the presence of the object held in pledge keeps the debt from being canceled by the Sabbatical year (Mitzvah #477). Also, if the borrower should pass away before the loan is repaid, the lender would have a claim on the collateral. There are other legal benefits to having an object as security. If the loan is not paid on time, beis din might sell the object to settle the debt.
The reason for this mitzvah is what we said: God wants us to treat one another well, not mistreat one another.
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 Shulchan Aruch in Choshen Mishpat 97. This mitzvah is #199 of the 248 positive mitzvos in the Rambam’s Sefer HaMitzvos and #63 of the 77 positive mitzvos that can be observed today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.