We have been warned several times about fair play when it comes to taking an object as security for a loan. For example, Mitzvah #583 forbids us from taking items needed for food preparation as a security, while Mitzvah #587 requires us to return the object held in security to its owner on an as-needed basis. Here, we are told that we may not take an object as a pledge from a widow, period. The Talmud in Baba Metzia (115a) spells out that it makes no difference if the widow is wealthy, a widow is a widow and we may not take a security from her.
The reason for this mitzvah is that the widow has had enough grief in her life, so we're not allowed to add to it. As with other laws pertaining to taking objects as security for a loan, it only means not to seize an object after the loan has been made in order to guarantee that it will be repaid. If the widow voluntary offers an object as collateral at the time the loan was made, it may be accepted by the lender.
This mitzvah applies in all times and places. It is discussed in the Talmud in tractate Baba Metzia on page 115a. It is codified in the Shulchan Aruch in Choshen Mishpat 97. This mitzvah is #241 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #60 of the 194 negative mitzvos that can be observed today as listed in the Chofetz Chaim’s Sefer HaMitzvos HaKatzar.