A micro bikini is an ultra-skimpy bathing suit. There are variations for both men and women, but both are designed to cover the barest amount of skin. Some are so sparse in design that they are probably illegal to wear on public beaches in some countries that don't allow nude sunbathing.
The micro bikini was first developed in the 1970s as a response to laws in some countries that made nude sunbathing illegal on public beaches. These early micro bikinis were designed to cover what the law required and nothing more. A woman's micro bikini might be little more than two very small pieces of fabric to hide the nipples and a skimpy, low-cut thong bottom. A man's version is simply nothing more than a very small thong bottom, with some displaying the full buttocks with just a small pouch up front. Some micro bikinis are even skimpier and don't even bother with strips of cloth. Instead they use fishing wire or other nearly-invisible materials to hold the incredibly small pieces of fabric in place.
Some versions of the micro bikini don't even bother to completely cover the genitalia. They may be so low cut in the front that the wearer is exposing themselves, or they may even be partially transparent. Others are specifically designed not to cover the breasts or genitalia. Some brands of male micro bikinis are just "socks" designed to cover the male parts in a form-fitting fashion.
Many public swimming pools and beaches across the world have laws against micro bikinis, even if they are covering the important parts. Most public pools or any other public place where children are allowed will typically have rules against g-strings, let alone micro mini bikinis that are even more revealing than the typical thong. Private pools and beaches are typically where people can wear micro string bikinis in these countries. Their ultra-revealing nature also makes them very popular on adult websites and in adult films as well.
One place where micro bikinis typically are commonplace and acceptable are at bodybuilding competitions. Since body builders are trying to show off as much of their muscles as possible, many times they will wear these incredibly revealing micro bikini swimsuits. In this instance, the micro bikini is not used as a way to titillate or attract people. It is simply a means to an end for both male and female bodybuilders to show as much of their definition and tone as possible.