Oxalis are grown from small bulbs that produce cute little white/pink/purple flowers that rise above the leaves. They have triangular shaped leaves that are grouped in threes and come in a variety of colors. During the night, their leaves fold up, and then open up in the daytime.
They may go through a dormancy phase where all of the leaves die off, but this is completely normal. It may last for a month or two. During this time, reduce watering and stop fertilizing until you see new growth.
Water when the top 1-2 inches of soil are dry. Place in bright, indirect light, an eastern or western facing window is best.