There are multiple meanings behind the bindi. According to Sanskritmagazine.com, for a married woman, wearing a bindi "signifies true love and prosperity." As well, there is spiritual significance behind the bindi. The place where a person wear a bindi (betwen the eyebrows), is known as the Ajna Chakra or "Third Eye". Featured below are some images of Hindu women wearing bindis.
A common Coachella fashion staple, bindis have become a popular accessory for many white and/or non-Hindu Western women. In popular fashion, the bindi is usually used to contribute to a "bohemian" look or style. Gwen Stefani is largely credited with popularizing this trend back in the 90s, but many more celebrites have been taking part since then. Some examples include Katy Perry, Selena Gomez, and Vanessa Hudgens, pictured below.
This is not just a celebrity occurence, however. Many non-celebrities have also participated in this fashion trend.